feat(ivy): able to compile @angular/core with ngtsc (#24677)

@angular/core is unique in that it defines the Angular decorators
(@Component, @Directive, etc). Ordinarily ngtsc looks for imports
from @angular/core in order to identify these decorators. Clearly
within core itself, this strategy doesn't work.

Instead, a special constant ITS_JUST_ANGULAR is declared within a
known file in @angular/core. If ngtsc sees this constant it knows
core is being compiled and can ignore the imports when evaluating
decorators.

Additionally, when compiling decorators ngtsc will often write an
import to @angular/core for needed symbols. However @angular/core
cannot import itself. This change creates a module within core to
export all the symbols needed to compile it and adds intelligence
within ngtsc to write relative imports to that module, instead of
absolute imports to @angular/core.

PR Close #24677

packages/compiler-cli/src/ngtsc/annotations/src/component.ts

@@ -25,10 +25,11 @@ const EMPTY_MAP = new Map<string, Expression>();
 export class ComponentDecoratorHandler implements DecoratorHandler<R3ComponentMetadata> {
   constructor(
       private checker: ts.TypeChecker, private reflector: ReflectionHost,
-      private scopeRegistry: SelectorScopeRegistry) {}
+      private scopeRegistry: SelectorScopeRegistry, private isCore: boolean) {}
 
   detect(decorators: Decorator[]): Decorator|undefined {
-    return decorators.find(decorator => decorator.name === 'Component' && isAngularCore(decorator));
+    return decorators.find(
+        decorator => decorator.name === 'Component' && (this.isCore || isAngularCore(decorator)));
   }
 
   analyze(node: ts.ClassDeclaration, decorator: Decorator): AnalysisOutput<R3ComponentMetadata> {
@@ -43,7 +44,7 @@ export class ComponentDecoratorHandler implements DecoratorHandler<R3ComponentMe
     // @Component inherits @Directive, so begin by extracting the @Directive metadata and building
     // on it.
     const directiveMetadata =
-        extractDirectiveMetadata(node, decorator, this.checker, this.reflector);
+        extractDirectiveMetadata(node, decorator, this.checker, this.reflector, this.isCore);
     if (directiveMetadata === undefined) {
       // `extractDirectiveMetadata` returns undefined when the @Directive has `jit: true`. In this
       // case, compilation of the decorator is skipped. Returning an empty object signifies

packages/compiler-cli/src/ngtsc/annotations/src/directive.ts

@@ -22,14 +22,16 @@ const EMPTY_OBJECT: {[key: string]: string} = {};
 export class DirectiveDecoratorHandler implements DecoratorHandler<R3DirectiveMetadata> {
   constructor(
       private checker: ts.TypeChecker, private reflector: ReflectionHost,
-      private scopeRegistry: SelectorScopeRegistry) {}
+      private scopeRegistry: SelectorScopeRegistry, private isCore: boolean) {}
 
   detect(decorators: Decorator[]): Decorator|undefined {
-    return decorators.find(decorator => decorator.name === 'Directive' && isAngularCore(decorator));
+    return decorators.find(
+        decorator => decorator.name === 'Directive' && (this.isCore || isAngularCore(decorator)));
   }
 
   analyze(node: ts.ClassDeclaration, decorator: Decorator): AnalysisOutput<R3DirectiveMetadata> {
-    const analysis = extractDirectiveMetadata(node, decorator, this.checker, this.reflector);
+    const analysis =
+        extractDirectiveMetadata(node, decorator, this.checker, this.reflector, this.isCore);
 
     // If the directive has a selector, it should be registered with the `SelectorScopeRegistry` so
     // when this directive appears in an `@NgModule` scope, its selector can be determined.
@@ -57,7 +59,7 @@ export class DirectiveDecoratorHandler implements DecoratorHandler<R3DirectiveMe
  */
 export function extractDirectiveMetadata(
     clazz: ts.ClassDeclaration, decorator: Decorator, checker: ts.TypeChecker,
-    reflector: ReflectionHost): R3DirectiveMetadata|undefined {
+    reflector: ReflectionHost, isCore: boolean): R3DirectiveMetadata|undefined {
   if (decorator.args === null || decorator.args.length !== 1) {
     throw new Error(`Incorrect number of arguments to @${decorator.name} decorator`);
   }
@@ -108,7 +110,7 @@ export function extractDirectiveMetadata(
 
   return {
     name: clazz.name !.text,
-    deps: getConstructorDependencies(clazz, reflector),
+    deps: getConstructorDependencies(clazz, reflector, isCore),
     host: {
       attributes: {},
       listeners: {},

packages/compiler-cli/src/ngtsc/annotations/src/injectable.ts

@@ -20,15 +20,16 @@ import {getConstructorDependencies, isAngularCore} from './util';
  * Adapts the `compileIvyInjectable` compiler for `@Injectable` decorators to the Ivy compiler.
  */
 export class InjectableDecoratorHandler implements DecoratorHandler<R3InjectableMetadata> {
-  constructor(private reflector: ReflectionHost) {}
+  constructor(private reflector: ReflectionHost, private isCore: boolean) {}
 
   detect(decorator: Decorator[]): Decorator|undefined {
-    return decorator.find(decorator => decorator.name === 'Injectable' && isAngularCore(decorator));
+    return decorator.find(
+        decorator => decorator.name === 'Injectable' && (this.isCore || isAngularCore(decorator)));
   }
 
   analyze(node: ts.ClassDeclaration, decorator: Decorator): AnalysisOutput<R3InjectableMetadata> {
     return {
-      analysis: extractInjectableMetadata(node, decorator, this.reflector),
+      analysis: extractInjectableMetadata(node, decorator, this.reflector, this.isCore),
     };
   }
 
@@ -48,8 +49,8 @@ export class InjectableDecoratorHandler implements DecoratorHandler<R3Injectable
  * metadata needed to run `compileIvyInjectable`.
  */
 function extractInjectableMetadata(
-    clazz: ts.ClassDeclaration, decorator: Decorator,
-    reflector: ReflectionHost): R3InjectableMetadata {
+    clazz: ts.ClassDeclaration, decorator: Decorator, reflector: ReflectionHost,
+    isCore: boolean): R3InjectableMetadata {
   if (clazz.name === undefined) {
     throw new Error(`@Injectables must have names`);
   }
@@ -63,7 +64,7 @@ function extractInjectableMetadata(
       name,
       type,
       providedIn: new LiteralExpr(null),
-      deps: getConstructorDependencies(clazz, reflector),
+      deps: getConstructorDependencies(clazz, reflector, isCore),
     };
   } else if (decorator.args.length === 1) {
     const metaNode = decorator.args[0];
@@ -102,7 +103,7 @@ function extractInjectableMetadata(
       }
       return {name, type, providedIn, useFactory: factory, deps};
     } else {
-      const deps = getConstructorDependencies(clazz, reflector);
+      const deps = getConstructorDependencies(clazz, reflector, isCore);
       return {name, type, providedIn, deps};
     }
   } else {

packages/compiler-cli/src/ngtsc/annotations/src/ng_module.ts

@@ -29,10 +29,11 @@ export interface NgModuleAnalysis {
 export class NgModuleDecoratorHandler implements DecoratorHandler<NgModuleAnalysis> {
   constructor(
       private checker: ts.TypeChecker, private reflector: ReflectionHost,
-      private scopeRegistry: SelectorScopeRegistry) {}
+      private scopeRegistry: SelectorScopeRegistry, private isCore: boolean) {}
 
   detect(decorators: Decorator[]): Decorator|undefined {
-    return decorators.find(decorator => decorator.name === 'NgModule' && isAngularCore(decorator));
+    return decorators.find(
+        decorator => decorator.name === 'NgModule' && (this.isCore || isAngularCore(decorator)));
   }
 
   analyze(node: ts.ClassDeclaration, decorator: Decorator): AnalysisOutput<NgModuleAnalysis> {
@@ -89,7 +90,7 @@ export class NgModuleDecoratorHandler implements DecoratorHandler<NgModuleAnalys
     const ngInjectorDef: R3InjectorMetadata = {
       name: node.name !.text,
       type: new WrappedNodeExpr(node.name !),
-      deps: getConstructorDependencies(node, this.reflector), providers,
+      deps: getConstructorDependencies(node, this.reflector, this.isCore), providers,
       imports: new LiteralArrayExpr(
           [...imports, ...exports].map(imp => referenceToExpression(imp, context))),
     };

packages/compiler-cli/src/ngtsc/annotations/src/util.ts

@@ -13,14 +13,15 @@ import {Decorator, ReflectionHost} from '../../host';
 import {Reference} from '../../metadata';
 
 export function getConstructorDependencies(
-    clazz: ts.ClassDeclaration, reflector: ReflectionHost): R3DependencyMetadata[] {
+    clazz: ts.ClassDeclaration, reflector: ReflectionHost,
+    isCore: boolean): R3DependencyMetadata[] {
   const useType: R3DependencyMetadata[] = [];
   const ctorParams = reflector.getConstructorParameters(clazz) || [];
   ctorParams.forEach((param, idx) => {
     let tokenExpr = param.type;
     let optional = false, self = false, skipSelf = false, host = false;
     let resolved = R3ResolvedDependencyType.Token;
-    (param.decorators || []).filter(isAngularCore).forEach(dec => {
+    (param.decorators || []).filter(dec => isCore || isAngularCore(dec)).forEach(dec => {
       if (dec.name === 'Inject') {
         if (dec.args === null || dec.args.length !== 1) {
           throw new Error(`Unexpected number of arguments to @Inject().`);

packages/compiler-cli/src/ngtsc/metadata/BUILD.bazel

@@ -13,5 +13,6 @@ ts_library(
         "//packages:types",
         "//packages/compiler",
         "//packages/compiler-cli/src/ngtsc/host",
+        "//packages/compiler-cli/src/ngtsc/util",
     ],
 )

packages/compiler-cli/src/ngtsc/program.ts

@@ -91,17 +91,21 @@ export class NgtscProgram implements api.Program {
     const mergeEmitResultsCallback = opts && opts.mergeEmitResultsCallback || mergeEmitResults;
 
     const checker = this.tsProgram.getTypeChecker();
+    const isCore = isAngularCorePackage(this.tsProgram);
     const reflector = new TypeScriptReflectionHost(checker);
     const scopeRegistry = new SelectorScopeRegistry(checker, reflector);
 
     // Set up the IvyCompilation, which manages state for the Ivy transformer.
     const handlers = [
-      new ComponentDecoratorHandler(checker, reflector, scopeRegistry),
-      new DirectiveDecoratorHandler(checker, reflector, scopeRegistry),
-      new InjectableDecoratorHandler(reflector),
-      new NgModuleDecoratorHandler(checker, reflector, scopeRegistry),
+      new ComponentDecoratorHandler(checker, reflector, scopeRegistry, isCore),
+      new DirectiveDecoratorHandler(checker, reflector, scopeRegistry, isCore),
+      new InjectableDecoratorHandler(reflector, isCore),
+      new NgModuleDecoratorHandler(checker, reflector, scopeRegistry, isCore),
     ];
-    const compilation = new IvyCompilation(handlers, checker, reflector);
+
+    const coreImportsFrom = isCore && getR3SymbolsFile(this.tsProgram) || null;
+
+    const compilation = new IvyCompilation(handlers, checker, reflector, coreImportsFrom);
 
     // Analyze every source file in the program.
     this.tsProgram.getSourceFiles()
@@ -115,7 +119,7 @@ export class NgtscProgram implements api.Program {
          sourceFiles: ReadonlyArray<ts.SourceFile>) => {
           if (fileName.endsWith('.d.ts')) {
             data = sourceFiles.reduce(
-                (data, sf) => compilation.transformedDtsFor(sf.fileName, data), data);
+                (data, sf) => compilation.transformedDtsFor(sf.fileName, data, fileName), data);
           }
           this.host.writeFile(fileName, data, writeByteOrderMark, onError, sourceFiles);
         };
@@ -128,7 +132,7 @@ export class NgtscProgram implements api.Program {
       options: this.options,
       emitOnlyDtsFiles: false, writeFile,
       customTransformers: {
-        before: [ivyTransformFactory(compilation)],
+        before: [ivyTransformFactory(compilation, coreImportsFrom)],
       },
     });
     return emitResult;
@@ -152,3 +156,47 @@ function mergeEmitResults(emitResults: ts.EmitResult[]): ts.EmitResult {
   }
   return {diagnostics, emitSkipped, emittedFiles};
 }
+
+/**
+ * Find the 'r3_symbols.ts' file in the given `Program`, or return `null` if it wasn't there.
+ */
+function getR3SymbolsFile(program: ts.Program): ts.SourceFile|null {
+  return program.getSourceFiles().find(file => file.fileName.indexOf('r3_symbols.ts') >= 0) || null;
+}
+
+/**
+ * Determine if the given `Program` is @angular/core.
+ */
+function isAngularCorePackage(program: ts.Program): boolean {
+  // Look for its_just_angular.ts somewhere in the program.
+  const r3Symbols = getR3SymbolsFile(program);
+  if (r3Symbols === null) {
+    return false;
+  }
+
+  // Look for the constant ITS_JUST_ANGULAR in that file.
+  return r3Symbols.statements.some(stmt => {
+    // The statement must be a variable declaration statement.
+    if (!ts.isVariableStatement(stmt)) {
+      return false;
+    }
+    // It must be exported.
+    if (stmt.modifiers === undefined ||
+        !stmt.modifiers.some(mod => mod.kind === ts.SyntaxKind.ExportKeyword)) {
+      return false;
+    }
+    // It must declare ITS_JUST_ANGULAR.
+    return stmt.declarationList.declarations.some(decl => {
+      // The declaration must match the name.
+      if (!ts.isIdentifier(decl.name) || decl.name.text !== 'ITS_JUST_ANGULAR') {
+        return false;
+      }
+      // It must initialize the variable to true.
+      if (decl.initializer === undefined || decl.initializer.kind !== ts.SyntaxKind.TrueKeyword) {
+        return false;
+      }
+      // This definition matches.
+      return true;
+    });
+  });
+}

packages/compiler-cli/src/ngtsc/transform/src/compilation.ts

@@ -46,9 +46,18 @@ export class IvyCompilation {
    */
   private dtsMap = new Map<string, DtsFileTransformer>();
 
+  /**
+   * @param handlers array of `DecoratorHandler`s which will be executed against each class in the
+   * program
+   * @param checker TypeScript `TypeChecker` instance for the program
+   * @param reflector `ReflectionHost` through which all reflection operations will be performed
+   * @param coreImportsFrom a TypeScript `SourceFile` which exports symbols needed for Ivy imports
+   * when compiling @angular/core, or `null` if the current program is not @angular/core. This is
+   * `null` in most cases.
+   */
   constructor(
       private handlers: DecoratorHandler<any>[], private checker: ts.TypeChecker,
-      private reflector: ReflectionHost) {}
+      private reflector: ReflectionHost, private coreImportsFrom: ts.SourceFile|null) {}
 
   /**
    * Analyze a source file and produce diagnostics for it (if any).
@@ -147,19 +156,19 @@ export class IvyCompilation {
    * Process a .d.ts source string and return a transformed version that incorporates the changes
    * made to the source file.
    */
-  transformedDtsFor(tsFileName: string, dtsOriginalSource: string): string {
+  transformedDtsFor(tsFileName: string, dtsOriginalSource: string, dtsPath: string): string {
     // No need to transform if no changes have been requested to the input file.
     if (!this.dtsMap.has(tsFileName)) {
       return dtsOriginalSource;
     }
 
     // Return the transformed .d.ts source.
-    return this.dtsMap.get(tsFileName) !.transform(dtsOriginalSource);
+    return this.dtsMap.get(tsFileName) !.transform(dtsOriginalSource, tsFileName);
   }
 
   private getDtsTransformer(tsFileName: string): DtsFileTransformer {
     if (!this.dtsMap.has(tsFileName)) {
-      this.dtsMap.set(tsFileName, new DtsFileTransformer());
+      this.dtsMap.set(tsFileName, new DtsFileTransformer(this.coreImportsFrom));
     }
     return this.dtsMap.get(tsFileName) !;
   }

packages/compiler-cli/src/ngtsc/transform/src/declaration.ts

@@ -8,6 +8,8 @@
 
 import * as ts from 'typescript';
 
+import {relativePathBetween} from '../../util/src/path';
+
 import {CompileResult} from './api';
 import {ImportManager, translateType} from './translator';
 
@@ -18,7 +20,11 @@ import {ImportManager, translateType} from './translator';
  */
 export class DtsFileTransformer {
   private ivyFields = new Map<string, CompileResult[]>();
-  private imports = new ImportManager();
+  private imports: ImportManager;
+
+  constructor(private coreImportsFrom: ts.SourceFile|null) {
+    this.imports = new ImportManager(coreImportsFrom !== null);
+  }
 
   /**
    * Track that a static field was added to the code for a class.
@@ -28,7 +34,7 @@ export class DtsFileTransformer {
   /**
    * Process the .d.ts text for a file and add any declarations which were recorded.
    */
-  transform(dts: string): string {
+  transform(dts: string, tsPath: string): string {
     const dtsFile =
         ts.createSourceFile('out.d.ts', dts, ts.ScriptTarget.Latest, false, ts.ScriptKind.TS);
 
@@ -51,7 +57,7 @@ export class DtsFileTransformer {
       }
     }
 
-    const imports = this.imports.getAllImports();
+    const imports = this.imports.getAllImports(tsPath, this.coreImportsFrom);
     if (imports.length !== 0) {
       dts = imports.map(i => `import * as ${i.as} from '${i.name}';\n`).join() + dts;
     }

packages/compiler-cli/src/ngtsc/transform/src/transform.ts

@@ -15,11 +15,12 @@ import {CompileResult} from './api';
 import {IvyCompilation} from './compilation';
 import {ImportManager, translateExpression, translateStatement} from './translator';
 
-export function ivyTransformFactory(compilation: IvyCompilation):
-    ts.TransformerFactory<ts.SourceFile> {
+export function ivyTransformFactory(
+    compilation: IvyCompilation,
+    coreImportsFrom: ts.SourceFile | null): ts.TransformerFactory<ts.SourceFile> {
   return (context: ts.TransformationContext): ts.Transformer<ts.SourceFile> => {
     return (file: ts.SourceFile): ts.SourceFile => {
-      return transformIvySourceFile(compilation, context, file);
+      return transformIvySourceFile(compilation, context, coreImportsFrom, file);
     };
   };
 }
@@ -74,18 +75,19 @@ class IvyVisitor extends Visitor {
  */
 function transformIvySourceFile(
     compilation: IvyCompilation, context: ts.TransformationContext,
-    file: ts.SourceFile): ts.SourceFile {
-  const importManager = new ImportManager();
+    coreImportsFrom: ts.SourceFile | null, file: ts.SourceFile): ts.SourceFile {
+  const importManager = new ImportManager(coreImportsFrom !== null);
 
   // Recursively scan through the AST and perform any updates requested by the IvyCompilation.
   const sf = visit(file, new IvyVisitor(compilation, importManager), context);
 
   // Generate the import statements to prepend.
-  const imports = importManager.getAllImports().map(
-      i => ts.createImportDeclaration(
-          undefined, undefined,
-          ts.createImportClause(undefined, ts.createNamespaceImport(ts.createIdentifier(i.as))),
-          ts.createLiteral(i.name)));
+  const imports = importManager.getAllImports(file.fileName, coreImportsFrom).map(i => {
+    return ts.createImportDeclaration(
+        undefined, undefined,
+        ts.createImportClause(undefined, ts.createNamespaceImport(ts.createIdentifier(i.as))),
+        ts.createLiteral(i.name));
+  });
 
   // Prepend imports if needed.
   if (imports.length > 0) {

packages/compiler-cli/src/ngtsc/transform/src/translator.ts

@@ -8,6 +8,7 @@
 
 import {ArrayType, AssertNotNull, BinaryOperator, BinaryOperatorExpr, BuiltinType, BuiltinTypeName, CastExpr, ClassStmt, CommaExpr, CommentStmt, ConditionalExpr, DeclareFunctionStmt, DeclareVarStmt, Expression, ExpressionStatement, ExpressionType, ExpressionVisitor, ExternalExpr, ExternalReference, FunctionExpr, IfStmt, InstantiateExpr, InvokeFunctionExpr, InvokeMethodExpr, JSDocCommentStmt, LiteralArrayExpr, LiteralExpr, LiteralMapExpr, MapType, NotExpr, ReadKeyExpr, ReadPropExpr, ReadVarExpr, ReturnStatement, Statement, StatementVisitor, ThrowStmt, TryCatchStmt, Type, TypeVisitor, WrappedNodeExpr, WriteKeyExpr, WritePropExpr, WriteVarExpr} from '@angular/compiler';
 import * as ts from 'typescript';
+import {relativePathBetween} from '../../util/src/path';
 
 const BINARY_OPERATORS = new Map<BinaryOperator, ts.BinaryOperator>([
   [BinaryOperator.And, ts.SyntaxKind.AmpersandAmpersandToken],
@@ -28,20 +29,44 @@ const BINARY_OPERATORS = new Map<BinaryOperator, ts.BinaryOperator>([
   [BinaryOperator.Plus, ts.SyntaxKind.PlusToken],
 ]);
 
+const CORE_SUPPORTED_SYMBOLS = new Set<string>([
+  'defineInjectable',
+  'defineInjector',
+  'ɵdefineNgModule',
+  'inject',
+  'InjectableDef',
+  'InjectorDef',
+  'NgModuleDef',
+]);
+
 export class ImportManager {
   private moduleToIndex = new Map<string, string>();
   private nextIndex = 0;
 
-  generateNamedImport(moduleName: string): string {
+  constructor(private isCore: boolean) {}
+
+  generateNamedImport(moduleName: string, symbol: string): string {
     if (!this.moduleToIndex.has(moduleName)) {
       this.moduleToIndex.set(moduleName, `i${this.nextIndex++}`);
     }
+    if (this.isCore && moduleName === '@angular/core' && !CORE_SUPPORTED_SYMBOLS.has(symbol)) {
+      throw new Error(`Importing unexpected symbol ${symbol} while compiling core`);
+    }
     return this.moduleToIndex.get(moduleName) !;
   }
 
-  getAllImports(): {name: string, as: string}[] {
+  getAllImports(contextPath: string, rewriteCoreImportsTo: ts.SourceFile|null):
+      {name: string, as: string}[] {
     return Array.from(this.moduleToIndex.keys()).map(name => {
-      const as = this.moduleToIndex.get(name) !;
+      const as: string|null = this.moduleToIndex.get(name) !;
+      if (rewriteCoreImportsTo !== null && name === '@angular/core') {
+        const relative = relativePathBetween(contextPath, rewriteCoreImportsTo.fileName);
+        if (relative === null) {
+          throw new Error(
+              `Failed to rewrite import inside core: ${contextPath} -> ${rewriteCoreImportsTo.fileName}`);
+        }
+        name = relative;
+      }
       return {name, as};
     });
   }
@@ -166,7 +191,7 @@ class ExpressionTranslatorVisitor implements ExpressionVisitor, StatementVisitor
       throw new Error(`Import unknown module or symbol ${ast.value}`);
     }
     return ts.createPropertyAccess(
-        ts.createIdentifier(this.imports.generateNamedImport(ast.value.moduleName)),
+        ts.createIdentifier(this.imports.generateNamedImport(ast.value.moduleName, ast.value.name)),
         ts.createIdentifier(ast.value.name));
   }
 
@@ -314,7 +339,8 @@ export class TypeTranslatorVisitor implements ExpressionVisitor, TypeVisitor {
     if (ast.value.moduleName === null || ast.value.name === null) {
       throw new Error(`Import unknown module or symbol`);
     }
-    const base = `${this.imports.generateNamedImport(ast.value.moduleName)}.${ast.value.name}`;
+    const moduleSymbol = this.imports.generateNamedImport(ast.value.moduleName, ast.value.name);
+    const base = `${moduleSymbol}.${ast.value.name}`;
     if (ast.typeParams !== null) {
       const generics = ast.typeParams.map(type => type.visitType(this, context)).join(', ');
       return `${base}<${generics}>`;

packages/compiler-cli/src/ngtsc/util/BUILD.bazel

@@ -9,4 +9,7 @@ ts_library(
         "src/**/*.ts",
     ]),
     module_name = "@angular/compiler-cli/src/ngtsc/util",
+    deps = [
+        "//packages:types",
+    ],
 )

packages/compiler-cli/src/ngtsc/util/src/path.ts

@@ -0,0 +1,26 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import * as path from 'path';
+
+const TS_DTS_EXTENSION = /(\.d)?\.ts$/;
+
+export function relativePathBetween(from: string, to: string): string|null {
+  let relative = path.posix.relative(path.dirname(from), to).replace(TS_DTS_EXTENSION, '');
+
+  if (relative === '') {
+    return null;
+  }
+
+  // path.relative() does not include the leading './'.
+  if (!relative.startsWith('.')) {
+    relative = `./${relative}`;
+  }
+
+  return relative;
+}

packages/core/src/r3_symbols.ts

@@ -0,0 +1,32 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/*
+ * This file exists to support compilation of @angular/core in Ivy mode.
+ *
+ * When the Angular compiler processes a compilation unit, it normally writes imports to
+ * @angular/core. When compiling the core package itself this strategy isn't usable. Instead, the
+ * compiler writes imports to this file.
+ *
+ * Only a subset of such imports are supported - core is not allowed to declare components or pipes.
+ * A check in ngtsc's translator.ts validates this condition.
+ *
+ * The below symbols are used for @Injectable and @NgModule compilation.
+ */
+
+export {InjectableDef, InjectorDef, defineInjectable, defineInjector} from './di/defs';
+export {inject} from './di/injector';
+export {NgModuleDef} from './metadata/ng_module';
+export {defineNgModule as ɵdefineNgModule} from './render3/definition';
+
+
+/**
+ * The existence of this constant (in this particular file) informs the Angular compiler that the
+ * current program is actually @angular/core, which needs to be compiled specially.
+ */
+export const ITS_JUST_ANGULAR = true;