anchor.diff:    ///   signature of such a method.
     /// - It has a non-nullable type variable return type, for example `T m<T>()`.
     ///   Mockito cannot generate dummy return values for unknown types.
  -  void _checkMethodsToStubAreValid(
  -      ClassElement classElement, LibraryElement entryLib) {
  +  void _checkMethodsToStubAreValid(ClassElement classElement) {
       var className = classElement.name;
  -    //var unstubbableErrorMessages = <String>[];
  -
  -    /*for (var method in classElement.methods) {
  -      if (method.isPrivate || method.isStatic) continue;
  -      _checkFunction(method.type, method.name, entryLib);
  -    }*/
       var unstubbableErrorMessages = classElement.methods
           .where((m) => !m.isPrivate && !m.isStatic)
  -        .expand((m) => _checkFunction(m.type, m.name, className, entryLib))
  +        .expand((m) => _checkFunction(m.type, m.name, className))
           .toList();
   
       if (unstubbableErrorMessages.isNotEmpty) {
  
positive.code:     return classesToMock;
    }
  
    void _checkClassesToMockAreValid(
        List<analyzer.DartType> classesToMock, LibraryElement entryLib) {
      var classesInEntryLib = entryLib.topLevelElements.whereType<ClassElement>();
      var classNamesToMock = <String, ClassElement>{};
      for (var class_ in classesToMock) {
        var name = class_.element.name;
        if (classNamesToMock.containsKey(name)) {
          var firstSource = classNamesToMock[name].source.fullName;
  
negative.code:                 'type, and cannot be stubbed.');
        }
      } else if (returnType is analyzer.FunctionType) {
        errorMessages.addAll(_checkFunction(returnType, name, className));
      } else if (returnType is analyzer.TypeParameterType) {
        if (function.returnType is analyzer.TypeParameterType &&
            _entryLib.typeSystem.isPotentiallyNonNullable(function.returnType)) {
          errorMessages
              .add("The method '$className.$name' features a non-nullable "
                  'unknown return type, and cannot be stubbed.');
  