feat(compiler-cli): lower metadata useValue and data literal fields (#18905)

With this commit the compiler will "lower" expressions into exported
variables for values the compiler does not need to know statically
in order to be able to generate a factory. For example:

```
  providers: [{provider: 'token', useValue: calculated()}]
```

produced an error as the expression `calculated()` is not supported
by the compiler because `calculated` is not a
[known function](https://angular.io/guide/metadata#annotationsdecorators)

With this commit this is rewritten, during emit of the .js file, into
something like:

```
export var ɵ0 = calculated();

  ...

  provdiers: [{provider: 'token', useValue: ɵ0}]
```

The compiler then will now generate a reference to the exported `ɵ0`
instead of failing to evaluate `calculated()`.

PR Close #18905

packages/compiler/src/output/ts_emitter.ts

@@ -35,12 +35,14 @@ export function debugOutputAstAsTypeScript(ast: o.Statement | o.Expression | o.T
   return ctx.toSource();
 }
 
+export type ReferenceFilter = (reference: o.ExternalReference) => boolean;
 
 export class TypeScriptEmitter implements OutputEmitter {
   emitStatementsAndContext(
       srcFilePath: string, genFilePath: string, stmts: o.Statement[], preamble: string = '',
-      emitSourceMaps: boolean = true): {sourceText: string, context: EmitterVisitorContext} {
-    const converter = new _TsEmitterVisitor();
+      emitSourceMaps: boolean = true,
+      referenceFilter?: ReferenceFilter): {sourceText: string, context: EmitterVisitorContext} {
+    const converter = new _TsEmitterVisitor(referenceFilter);
 
     const ctx = EmitterVisitorContext.createRoot();
 
@@ -78,10 +80,11 @@ export class TypeScriptEmitter implements OutputEmitter {
   }
 }
 
+
 class _TsEmitterVisitor extends AbstractEmitterVisitor implements o.TypeVisitor {
   private typeExpression = 0;
 
-  constructor() { super(false); }
+  constructor(private referenceFilter?: ReferenceFilter) { super(false); }
 
   importsWithPrefixes = new Map<string, string>();
   reexports = new Map<string, {name: string, as: string}[]>();
@@ -377,6 +380,10 @@ class _TsEmitterVisitor extends AbstractEmitterVisitor implements o.TypeVisitor
   private _visitIdentifier(
       value: o.ExternalReference, typeParams: o.Type[]|null, ctx: EmitterVisitorContext): void {
     const {name, moduleName} = value;
+    if (this.referenceFilter && this.referenceFilter(value)) {
+      ctx.print(null, '(null as any)');
+      return;
+    }
     if (moduleName) {
       let prefix = this.importsWithPrefixes.get(moduleName);
       if (prefix == null) {

packages/compiler/test/aot/test_util.ts

@@ -12,6 +12,8 @@ import * as fs from 'fs';
 import * as path from 'path';
 import * as ts from 'typescript';
 
+export interface MetadataProvider { getMetadata(source: ts.SourceFile): ModuleMetadata|undefined; }
+
 let nodeModulesPath: string;
 let angularSourcePath: string;
 let rootPath: string;
@@ -327,12 +329,13 @@ const DTS = /\.d\.ts$/;
 const GENERATED_FILES = /\.ngfactory\.ts$|\.ngstyle\.ts$/;
 
 export class MockAotCompilerHost implements AotCompilerHost {
-  private metadataCollector = new MetadataCollector();
   private metadataVisible: boolean = true;
   private dtsAreSource: boolean = true;
   private resolveModuleNameHost: ts.ModuleResolutionHost;
 
-  constructor(private tsHost: MockCompilerHost) {
+  constructor(
+      private tsHost: MockCompilerHost,
+      private metadataProvider: MetadataProvider = new MetadataCollector()) {
     this.resolveModuleNameHost = Object.create(tsHost);
     this.resolveModuleNameHost.fileExists = (fileName) => {
       fileName = stripNgResourceSuffix(fileName);
@@ -359,7 +362,7 @@ export class MockAotCompilerHost implements AotCompilerHost {
       }
     } else {
       const sf = this.tsHost.getSourceFile(modulePath, ts.ScriptTarget.Latest);
-      const metadata = this.metadataCollector.getMetadata(sf);
+      const metadata = this.metadataProvider.getMetadata(sf);
       return metadata ? [metadata] : [];
     }
     return undefined;