perf(Compiler): use Promises only when strictly required

modules/angular2/src/facade/async.dart

@@ -20,6 +20,10 @@ class PromiseWrapper {
   static void setTimeout(fn(), int millis) {
     new Timer(new Duration(milliseconds: millis), fn);
   }
+
+  static bool isPromise(maybePromise) {
+    return maybePromise is Future;
+  }
 }
 
 class _Completer {

modules/angular2/src/facade/async.es6

@@ -39,4 +39,8 @@ export class PromiseWrapper {
   static setTimeout(fn:Function, millis:int) {
     window.setTimeout(fn, millis);
   }
+
+  static isPromise(maybePromise):boolean {
+    return maybePromise instanceof Promise;
+  }
 }