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/style_compiler_spec.ts

@@ -25,7 +25,7 @@ import {
   isArray,
   IS_DART
 } from 'angular2/src/facade/lang';
-import {PromiseWrapper, Promise} from 'angular2/src/facade/async';
+import {PromiseWrapper} from 'angular2/src/facade/async';
 import {evalModule} from './eval_module';
 import {StyleCompiler} from 'angular2/src/compiler/style_compiler';
 import {