fix(forms): add support for radio buttons

Closes #6877

modules/angular2/test/public_api_spec.ts

@@ -51,6 +51,7 @@ var NG_COMMON = [
   'AbstractControl.validator',
   'AbstractControl.validator=',
   'AbstractControl.value',
+  'AbstractControl.root',
   'AbstractControl.valueChanges',
   'AbstractControlDirective',
   'AbstractControlDirective.control',
@@ -102,6 +103,7 @@ var NG_COMMON = [
   'Control.validator',
   'Control.validator=',
   'Control.value',
+  'Control.root',
   'Control.valueChanges',
   'ControlArray',
   'ControlArray.asyncValidator',
@@ -134,6 +136,7 @@ var NG_COMMON = [
   'ControlArray.validator',
   'ControlArray.validator=',
   'ControlArray.value',
+  'ControlArray.root',
   'ControlArray.valueChanges',
   'ControlContainer',
   'ControlContainer.control',
@@ -179,6 +182,7 @@ var NG_COMMON = [
   'ControlGroup.validator',
   'ControlGroup.validator=',
   'ControlGroup.value',
+  'ControlGroup.root',
   'ControlGroup.valueChanges',
   'ControlValueAccessor:dart',
   'CurrencyPipe',
@@ -447,7 +451,12 @@ var NG_COMMON = [
   'Validators#maxLength()',
   'Validators#minLength()',
   'Validators#nullValidator()',
-  'Validators#required()'
+  'Validators#required()',
+  'RadioButtonState',
+  'RadioButtonState.checked',
+  'RadioButtonState.checked=',
+  'RadioButtonState.value',
+  'RadioButtonState.value='
 ];
 
 var NG_COMPILER = [