feat(typings): mark void methods in angular2.d.ts

Previously, when a return type was missing it
could have been any. But following #2746 we
require return types so remaining untyped returns
must be void.

modules/angular2/src/forms/directives/ng_model.ts

@@ -63,7 +63,7 @@ export class NgModel extends NgControl {
     }
   }
 
-  get control() { return this._control; }
+  get control(): Control { return this._control; }
 
   get path(): List<string> { return []; }