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/compiler/eval_module.ts

@@ -1,4 +1,4 @@
-import {Promise, PromiseWrapper} from 'angular2/src/facade/async';
+import {PromiseWrapper} from 'angular2/src/facade/async';
 import {isPresent, global, StringWrapper} from 'angular2/src/facade/lang';
 
 var evalCounter = 0;