Mapping not getting called

Look for the `iit` test to test this out.

Goto `/lib/directive/ng_model_validators.dart` to line 120.

The mapping on the 2nd directive isn't getting called.
```dart
@NgDirective(selector: '[ng-model][pattern]')
@NgDirective(selector: '[ng-model][ng-pattern]', map: const {'ng-pattern': '=>pattern'})
```