fix(typings): repair broken typechecks

We had the typechecker disabled by accident, and many problems snuck in

Fixes #4507

Closes #4508

modules/angular2/test/core/forms/directives_spec.ts

@@ -93,7 +93,7 @@ export function main() {
         });
 
         it("should throw when more than one custom accessor is provided", () => {
-          var customAccessor = new SpyValueAccessor();
+          var customAccessor: ControlValueAccessor = <any>new SpyValueAccessor();
           expect(() => selectValueAccessor(dir, [customAccessor, customAccessor])).toThrowError();
         });
       });

modules/angular2/test/core/forms/model_spec.ts

@@ -156,7 +156,7 @@ export function main() {
               {"one": new Control("111"), "nested": new ControlGroup({"two": new Control("222")})});
           expect(g.value).toEqual({"one": "111", "nested": {"two": "222"}});
 
-          g.controls["nested"].controls["two"].updateValue("333");
+          (<Control>(g.controls["nested"].find("two"))).updateValue("333");
 
           expect(g.value).toEqual({"one": "111", "nested": {"two": "333"}});
         });