anchor.diff:  class MyWidget extends StatelessWidget {
     MyWidget(int a);
     MyWidget.named(int a);
  -  Widget build(BuildContext context) => null;
  +  Widget build(BuildContext context) => Text('');
   }
   ''');
       var f = testUnit.declarations[0] as FunctionDeclaration;
  
positive.code:   useWidget(child: text); // ref
  }
  
  void useWidget({Widget child}) {}
  ''');
      var expression = findNode.simple('text); // ref');
      expect(_flutter.identifyWidgetExpression(expression), expression);
  
negative.code: 
  @reflectiveTest
  class FlutterTest extends AbstractSingleUnitTest {
    Flutter get _flutter => Flutter.instance;
  
    @override
  