feat(facade): add a way to convert observables into promises

Author: vsavkin

modules/angular2/src/core/facade/async.dart

@@ -61,6 +61,10 @@ class ObservableWrapper {
   static Stream fromPromise(Future f) {
     return new Stream.fromFuture(f);
   }
+
+  static Future toPromise(Stream s) {
+    return s.single;
+  }
 }
 
 class EventEmitter<T> extends Stream<T> {

modules/angular2/src/core/facade/async.ts

@@ -48,6 +48,8 @@ export class ObservableWrapper {
   static fromPromise(promise: Promise<any>): Observable<any> {
     return RxObservable.fromPromise(promise);
   }
+
+  static toPromise(obj: Observable<any>): Promise<any> { return (<any>obj).toPromise(); }
 }
 
 /**