fix(Validators): fix Validators.required marking number zero as invalid

Closes #6617

modules/angular2/src/common/forms/validators.ts

@@ -1,4 +1,4 @@
-import {isBlank, isPresent, CONST_EXPR} from 'angular2/src/facade/lang';
+import {isBlank, isPresent, CONST_EXPR, isString} from 'angular2/src/facade/lang';
 import {PromiseWrapper} from 'angular2/src/facade/promise';
 import {ObservableWrapper} from 'angular2/src/facade/async';
 import {ListWrapper, StringMapWrapper} from 'angular2/src/facade/collection';
@@ -44,7 +44,9 @@ export class Validators {
    * Validator that requires controls to have a non-empty value.
    */
   static required(control: modelModule.Control): {[key: string]: boolean} {
-    return isBlank(control.value) || control.value == "" ? {"required": true} : null;
+    return isBlank(control.value) || (isString(control.value) && control.value == "") ?
+               {"required": true} :
+               null;
   }
 
   /**

modules/angular2/test/common/forms/validators_spec.ts

@@ -35,6 +35,9 @@ export function main() {
 
       it("should not error on a non-empty string",
          () => { expect(Validators.required(new Control("not empty"))).toEqual(null); });
+
+      it("should accept zero as valid",
+         () => { expect(Validators.required(new Control(0))).toEqual(null); });
     });
 
     describe("minLength", () => {