fix(typing): Remove re-export of the Promise built-in type.

Instead, ts2dart can add the 'dart:async' import whenever
Promise is used.

Fixes #6468

modules/angular2/test/facade/async_dart_spec.dart

@@ -1,6 +1,7 @@
 /// This file contains tests that make sense only in Dart
 library angular2.test.facade.async_dart_spec;
 
+import 'dart:async';
 import 'package:angular2/testing_internal.dart';
 import 'package:angular2/src/facade/async.dart';