anchor.diff:  
   abstract class Foo extends Widget {
     Widget bar;
  +
  +  Foo(this.bar);
   }
   
   void f(Foo foo) {
  
positive.code: 
  abstract class Foo extends Widget {
    Widget bar;
  }
  
  void f(Foo foo) {
  
negative.code: class MyWidget extends StatelessWidget {
    MyWidget(int a);
    MyWidget.named(int a);
    Widget build(BuildContext context) => Text('');
  }
  ''');
      var f = testUnit.declarations[0] as FunctionDeclaration;
  