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/util.ts

@@ -6,6 +6,8 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {ɵisPromise as isPromise} from '@angular/core';
+
 import * as o from './output/output_ast';
 
 export const MODULE_SUFFIX = '';
@@ -80,19 +82,28 @@ export class ValueTransformer implements ValueVisitor {
   visitOther(value: any, context: any): any { return value; }
 }
 
-export class SyncAsyncResult<T> {
-  constructor(public syncResult: T|null, public asyncResult: Promise<T>|null = null) {
-    if (!asyncResult) {
-      this.asyncResult = Promise.resolve(syncResult);
+export type SyncAsync<T> = T | Promise<T>;
+
+export const SyncAsync = {
+  assertSync: <T>(value: SyncAsync<T>): T => {
+    if (isPromise(value)) {
+      throw new Error(`Illegal state: value cannot be a promise`);
     }
+    return value;
+  },
+  then: <T, R>(value: SyncAsync<T>, cb: (value: T) => R | Promise<R>| SyncAsync<R>):
+            SyncAsync<R> => { return isPromise(value) ? value.then(cb) : cb(value);},
+  all: <T>(syncAsyncValues: SyncAsync<T>[]): SyncAsync<T[]> => {
+    return syncAsyncValues.some(isPromise) ? Promise.all(syncAsyncValues) : syncAsyncValues as T[];
   }
 }
 
-export function syntaxError(msg: string): Error {
-  const error = Error(msg);
-  (error as any)[ERROR_SYNTAX_ERROR] = true;
-  return error;
-}
+export function syntaxError(msg: string):
+    Error {
+      const error = Error(msg);
+      (error as any)[ERROR_SYNTAX_ERROR] = true;
+      return error;
+    }
 
 const ERROR_SYNTAX_ERROR = 'ngSyntaxError';