fix: improper use package name in facade

Closes angular#3613

Author: mhevery

modules/angular2/src/facade/async.dart

@@ -1,4 +1,4 @@
-library angular.core.facade.async;
+library angular2.core.facade.async;
 
 import 'dart:async';
 export 'dart:async' show Future, Stream, StreamController, StreamSubscription;