feat(decorators): adds decorator versions of DI annotations.

In 'angular2/di' the symbol:
- Inject is a decorator
- InjectAnnotation is an annotation

Internally one an get a hold of annotations without *Annotations appened
(to make ts2dart work without workarounds) by importing from
'angular2/src/di/annotations_impl' instead of 'angular2/di'. This is
needed only for users that transpile through TS and through ts2dart.

modules/angular2/src/forms/directives.js

@@ -1,7 +1,7 @@
 import {Directive, onChange} from 'angular2/src/core/annotations_impl/annotations';
 import {Ancestor} from 'angular2/src/core/annotations_impl/visibility';
 import {ElementRef} from 'angular2/src/core/compiler/element_ref';
-import {Optional} from 'angular2/di';
+import {Optional} from 'angular2/src/di/annotations_impl';
 import {Renderer} from 'angular2/src/render/api';
 import {isPresent, isString} from 'angular2/src/facade/lang';
 import {ListWrapper} from 'angular2/src/facade/collection';