anchor.diff:  }
   
   class TestDartObject {}
  +
  +class Callable {
  +  call() => 'called';
  +}
   
   main() {
     _injectJs();
  
positive.code: 
    });
  
    group('Dart callback', () {
      test('invoke Dart callback from JS', () {
        expect(() => context.callMethod('invokeCallback'), throws);
  
  
negative.code:     test('return a JS proxy to JavaScript', () {
        var result = context.callMethod('testJsMap', [() => new JsObject.jsify({'value': 42})]);
        expect(result, 42);
      });
  
      test('emulated functions should be callable in JS', () {
        context['callable'] = new Callable();
        var result = context.callMethod('callable');
        expect(result, 'called');
        context.deleteProperty('callable');
      });
  
    });
  