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/di.ts

@@ -7,6 +7,7 @@
 
 export * from './src/di/annotations';
 export * from './src/di/decorators';
+export * from './src/di/forward_ref';
 export {Injector} from './src/di/injector';
 export {Binding, ResolvedBinding, Dependency, bind} from './src/di/binding';
 export {Key, KeyRegistry, TypeLiteral} from './src/di/key';