anchor.diff:          unit, code, "() => 0)", (node) => node is FunctionExpression);
       expect((functionExpression.staticType as FunctionType).parameters.length,
           same(0));
  -    expect(functionExpression.propagatedType, same(null));
  +    expect(functionExpression.propagatedType, isNull);
     }
   
     test_functionExpression_asInvocationArgument_replaceIfMoreSpecific() async {
  
positive.code:     {
        SimpleIdentifier identifier = EngineTestCase.findNode(
            unit, code, "v = null;", (node) => node is SimpleIdentifier);
        expect(identifier.staticType, same(typeProvider.intType));
        expect(identifier.propagatedType, same(null));
      }
      {
        SimpleIdentifier identifier = EngineTestCase.findNode(
            unit, code, "v; // marker", (node) => node is SimpleIdentifier);
        expect(identifier.staticType, same(typeProvider.intType));
        expect(identifier.propagatedType, same(null));
      }
    }
  
  
negative.code:     Source source = addSource(code);
      CompilationUnit unit = await _computeResolvedUnit(source);
      // v
      FormalParameter vParameter = EngineTestCase.findNode(
          unit, code, "v)", (node) => node is SimpleFormalParameter);
      expect(vParameter.identifier.propagatedType, isNull);
      expect(vParameter.identifier.staticType, typeProvider.intType);
      SimpleIdentifier vIdentifier = EngineTestCase.findNode(
          unit, code, "v;", (node) => node is SimpleIdentifier);
      expect(vIdentifier.staticType, typeProvider.intType);
      expect(vIdentifier.propagatedType, isNull);
    }
  
    test_functionExpression_asInvocationArgument_notSubtypeOfStaticType() async {
  