refactor(compiler): emit OutputAst and not sources (#16832)

This is in preparation for creating typescript nodes
directly from `OutputAst` nodes.

packages/compiler/src/aot/generated_file.ts

@@ -6,6 +6,30 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {sourceUrl} from '../compile_metadata';
+import {Statement} from '../output/output_ast';
+import {TypeScriptEmitter} from '../output/ts_emitter';
+
 export class GeneratedFile {
-  constructor(public srcFileUrl: string, public genFileUrl: string, public source: string) {}
+  public source: string|null;
+  public stmts: Statement[]|null;
+
+  constructor(
+      public srcFileUrl: string, public genFileUrl: string, sourceOrStmts: string|Statement[]) {
+    if (typeof sourceOrStmts === 'string') {
+      this.source = sourceOrStmts;
+      this.stmts = null;
+    } else {
+      this.source = null;
+      this.stmts = sourceOrStmts;
+    }
+  }
+}
+
+export function toTypeScript(file: GeneratedFile, preamble: string = ''): string {
+  if (!file.stmts) {
+    throw new Error(`Illegal state: No stmts present on GeneratedFile ${file.genFileUrl}`);
+  }
+  return new TypeScriptEmitter().emitStatements(
+      sourceUrl(file.srcFileUrl), file.genFileUrl, file.stmts, preamble);
 }