feat: allow for forward references in injection

It is possible for a class defined first to be referencing a class defined later,
and as a result at the time of the definition it is not possible to access the later's
class reference. This allows to refer to the later defined class through
a closure.Closes #1891

modules/angular2/angular2.js

@@ -3,6 +3,7 @@ export * from './core';
 export * from './annotations';
 export * from './directives';
 export * from './forms';
+export * from './di';
 export {Observable, EventEmitter} from 'angular2/src/facade/async';
 export * from 'angular2/src/render/api';
 export {DomRenderer, DOCUMENT_TOKEN} from 'angular2/src/render/dom/dom_renderer';