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/examples/router/ts/reuse/reuse_spec.ts

@@ -1,5 +1,4 @@
 import {verifyNoBrowserErrors} from 'angular2/src/testing/e2e_util';
-import {Promise} from 'angular2/src/facade/async';
 
 function waitForElement(selector) {
   var EC = (<any>protractor).ExpectedConditions;