refactor(compiler): allow sync AOT compilation (#16832).

AOT compilation can be executed synchronously now,
if the `ReosurceLoader` returns a string directly
(and no `Promise`).

packages/compiler/src/resource_loader.ts

@@ -11,5 +11,5 @@
  * to load templates.
  */
 export class ResourceLoader {
-  get(url: string): Promise<string>|null { return null; }
+  get(url: string): Promise<string>|string { return ''; }
 }