fix(compiler): generate safe access strictNullChecks compatible code (#12800)

fixes #12795

modules/@angular/compiler/src/output/output_ast.ts

@@ -32,7 +32,8 @@ export enum BuiltinTypeName {
   String,
   Int,
   Number,
-  Function
+  Function,
+  Null
 }
 
 export class BuiltinType extends Type {
@@ -73,7 +74,7 @@ export var INT_TYPE = new BuiltinType(BuiltinTypeName.Int);
 export var NUMBER_TYPE = new BuiltinType(BuiltinTypeName.Number);
 export var STRING_TYPE = new BuiltinType(BuiltinTypeName.String);
 export var FUNCTION_TYPE = new BuiltinType(BuiltinTypeName.Function);
-
+export var NULL_TYPE = new BuiltinType(BuiltinTypeName.Null);
 
 export interface TypeVisitor {
   visitBuiltintType(type: BuiltinType, context: any): any;
@@ -175,7 +176,8 @@ export abstract class Expression {
   }
   isBlank(): Expression {
     // Note: We use equals by purpose here to compare to null and undefined in JS.
-    return this.equals(NULL_EXPR);
+    // We use the typed null to allow strictNullChecks to narrow types.
+    return this.equals(TYPED_NULL_EXPR);
   }
   cast(type: Type): Expression { return new CastExpr(this, type); }
   toStmt(): Statement { return new ExpressionStatement(this); }
@@ -451,6 +453,7 @@ export var SUPER_EXPR = new ReadVarExpr(BuiltinVar.Super);
 export var CATCH_ERROR_VAR = new ReadVarExpr(BuiltinVar.CatchError);
 export var CATCH_STACK_VAR = new ReadVarExpr(BuiltinVar.CatchStack);
 export var NULL_EXPR = new LiteralExpr(null, null);
+export var TYPED_NULL_EXPR = new LiteralExpr(null, NULL_TYPE);
 
 //// Statements
 export enum StmtModifier {

modules/@angular/compiler/src/output/ts_emitter.ts

@@ -69,7 +69,7 @@ class _TsEmitterVisitor extends AbstractEmitterVisitor implements o.TypeVisitor
 
   visitLiteralExpr(ast: o.LiteralExpr, ctx: EmitterVisitorContext): any {
     const value = ast.value;
-    if (isBlank(value)) {
+    if (isBlank(value) && ast.type != o.NULL_TYPE) {
       ctx.print(`(${value} as any)`);
       return null;
     }

modules/@angular/compiler/test/output/ts_emitter_spec.ts

@@ -110,6 +110,7 @@ export function main() {
     it('should support blank literals', () => {
       expect(emitStmt(o.literal(null).toStmt())).toEqual('(null as any);');
       expect(emitStmt(o.literal(undefined).toStmt())).toEqual('(undefined as any);');
+      expect(emitStmt(o.variable('a', null).isBlank().toStmt())).toEqual('(a == null);');
     });
 
     it('should support external identifiers', () => {