feat(forms): make NgControl -> NgValueAccessor dependency unidirectional

Closes #4421

modules/angular2/test/core/spies.ts

@@ -105,3 +105,7 @@ export class SpyRenderEventDispatcher extends SpyObject {
     this.spy('dispatchRenderEvent');
   }
 }
+
+export class SpyNgControl extends SpyObject {}
+
+export class SpyValueAccessor extends SpyObject {}