feat(facade): add a way to detect if an object is a Promise

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

@@ -34,6 +34,10 @@ class PromiseWrapper {
     async.scheduleMicrotask(fn);
   }
 
+  static bool isPromise(obj) {
+    return obj is Future;
+  }
+
   static PromiseCompleter<dynamic> completer() =>
       new PromiseCompleter(new Completer());
 }

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

@@ -44,6 +44,8 @@ export class PromiseWrapper {
     PromiseWrapper.then(PromiseWrapper.resolve(null), computation, (_) => {});
   }
 
+  static isPromise(obj: any): boolean { return obj instanceof Promise; }
+
   static completer(): PromiseCompleter<any> {
     var resolve;
     var reject;