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/change_detector_compiler_spec.ts

@@ -16,7 +16,6 @@ import {provide} from 'angular2/src/core/di';
 
 import {CONST_EXPR, stringify, IS_DART} from 'angular2/src/facade/lang';
 import {MapWrapper} from 'angular2/src/facade/collection';
-import {Promise} from 'angular2/src/facade/async';
 
 import {ChangeDetectionCompiler} from 'angular2/src/compiler/change_detector_compiler';