feat(async): added PromiseWrapper.wrap

modules/angular2/src/facade/async.dart

@@ -19,6 +19,10 @@ class PromiseWrapper {
     return promise.then(success, onError: onError);
   }
 
+  static Future wrap(Function fn) {
+    return new Future(fn);
+  }
+  
   // Note: We can't rename this method to `catch`, as this is not a valid
   // method name in Dart.
   static Future catchError(Future promise, Function onError) {