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/aot/compiler_host.ts

@@ -17,5 +17,5 @@ export interface AotCompilerHost extends StaticSymbolResolverHost, AotSummaryRes
   /**
    * Loads a resource (e.g. html / css)
    */
-  loadResource(path: string): Promise<string>;
+  loadResource(path: string): Promise<string>|string;
 }