fix(forms): use strict runtimeType checks instead of instanceof

Currently, validators extending built-in validators are treated as built-in.
This can result in an error when both a real built-in validator and a custom one are applied to the same element.

Closes #6981

modules/angular2/test/facade/lang_spec.ts

@@ -4,9 +4,13 @@ import {
   RegExpWrapper,
   RegExpMatcherWrapper,
   StringWrapper,
-  CONST_EXPR
+  CONST_EXPR,
+  hasConstructor
 } from 'angular2/src/facade/lang';
 
+class MySuperclass {}
+class MySubclass extends MySuperclass {}
+
 export function main() {
   describe('RegExp', () => {
     it('should expose the index for each match', () => {
@@ -119,5 +123,13 @@ export function main() {
         expect(StringWrapper.stripRight(null, "S")).toEqual(null);
       });
     });
+
+    describe('hasConstructor', () => {
+      it("should be true when the type matches",
+         () => { expect(hasConstructor(new MySuperclass(), MySuperclass)).toEqual(true); });
+
+      it("should be false for subtypes",
+         () => { expect(hasConstructor(new MySubclass(), MySuperclass)).toEqual(false); });
+    });
   });
 }