refactor(core): Introduce AppInitStatus

This class allows any provider to know and wait for the initialization of the
application. This functionality previously was tied to `ApplicationRef`.

BREAKING CHANGE:
- `ApplicationRef.waitForAsyncInitializers` is deprecated. Use 
  `AppInitStatus.donePromise` / `AppInitStatus.done` instead.

modules/@angular/core/src/application_init.ts

@@ -0,0 +1,49 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {isPromise} from '../src/facade/lang';
+
+import {Inject, Injectable, OpaqueToken, Optional} from './di';
+
+
+/**
+ * A function that will be executed when an application is initialized.
+ * @experimental
+ */
+export const APP_INITIALIZER: any = new OpaqueToken('Application Initializer');
+
+/**
+ * A class that reflects the state of running {@link APP_INITIALIZER}s.
+ *
+ * @experimental
+ */
+@Injectable()
+export class AppInitStatus {
+  private _donePromise: Promise<any>;
+  private _done = false;
+
+  constructor(@Inject(APP_INITIALIZER) @Optional() appInits: (() => any)[]) {
+    const asyncInitPromises: Promise<any>[] = [];
+    if (appInits) {
+      for (let i = 0; i < appInits.length; i++) {
+        const initResult = appInits[i]();
+        if (isPromise(initResult)) {
+          asyncInitPromises.push(initResult);
+        }
+      }
+    }
+    this._donePromise = Promise.all(asyncInitPromises).then(() => { this._done = true; });
+    if (asyncInitPromises.length === 0) {
+      this._done = true;
+    }
+  }
+
+  get done(): boolean { return this._done; }
+
+  get donePromise(): Promise<any> { return this._donePromise; }
+}