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/src/testing/test_component_builder.ts

@@ -12,7 +12,6 @@ import {
 } from 'angular2/core';
 
 import {Type, isPresent, isBlank} from 'angular2/src/facade/lang';
-import {Promise} from 'angular2/src/facade/async';
 import {ListWrapper, MapWrapper} from 'angular2/src/facade/collection';
 
 import {ViewRef_} from 'angular2/src/core/linker/view_ref';