fix(ngcc): sniff `main` property for ESM5 format (#36396)

Previously, `main` was only checked for `umd` or `commonjs`
formats. Now if there are `import` or `export` statements in the
source file it will be deemed to be in `esm5` format.

Fixes #35788

PR Close #36396

packages/compiler-cli/ngcc/src/packages/entry_point.ts

@@ -199,7 +199,7 @@ export function getEntryPointFormat(
         return undefined;
       }
       const pathToMain = join(entryPoint.path, mainFile);
-      return isUmdModule(fs, pathToMain) ? 'umd' : 'commonjs';
+      return sniffModuleFormat(fs, pathToMain);
     case 'module':
       return 'esm5';
     default:
@@ -226,15 +226,25 @@ function loadEntryPointPackage(
   }
 }
 
-function isUmdModule(fs: FileSystem, sourceFilePath: AbsoluteFsPath): boolean {
+function sniffModuleFormat(fs: FileSystem, sourceFilePath: AbsoluteFsPath): EntryPointFormat|
+    undefined {
   const resolvedPath = resolveFileWithPostfixes(fs, sourceFilePath, ['', '.js', '/index.js']);
   if (resolvedPath === null) {
-    return false;
+    return undefined;
   }
+
   const sourceFile =
       ts.createSourceFile(sourceFilePath, fs.readFile(resolvedPath), ts.ScriptTarget.ES5);
-  return sourceFile.statements.length > 0 &&
-      parseStatementForUmdModule(sourceFile.statements[0]) !== null;
+  if (sourceFile.statements.length === 0) {
+    return undefined;
+  }
+  if (ts.isExternalModule(sourceFile)) {
+    return 'esm5';
+  } else if (parseStatementForUmdModule(sourceFile.statements[0]) !== null) {
+    return 'umd';
+  } else {
+    return 'commonjs';
+  }
 }
 
 function mergeConfigAndPackageJson(

packages/compiler-cli/ngcc/test/packages/entry_point_spec.ts

@@ -425,6 +425,22 @@ runInEachFileSystem(() => {
       expect(getEntryPointFormat(fs, entryPoint, 'module')).toBe('esm5');
     });
 
+    it('should return `esm5` for `main` if the file contains import or export statements', () => {
+      const name = _(
+          '/project/node_modules/some_package/valid_entry_point/bundles/valid_entry_point/index.js');
+      loadTestFiles([{name, contents: `import * as core from '@angular/core;`}]);
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('esm5');
+
+      loadTestFiles([{name, contents: `import {Component} from '@angular/core;`}]);
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('esm5');
+
+      loadTestFiles([{name, contents: `export function foo() {}`}]);
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('esm5');
+
+      loadTestFiles([{name, contents: `export * from 'abc';`}]);
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('esm5');
+    });
+
     it('should return `umd` for `main` if the file contains a UMD wrapper function', () => {
       loadTestFiles([{
         name: _(