anchor.diff:              'appears to already be mocked inline: ${preexistingMock.name}');
         }
   
  -      _checkMethodsToStubAreValid(element, entryLib);
  +      _checkMethodsToStubAreValid(element);
       });
     }
   
  
positive.code: /// 'foo.mocks.dart' will be created.
  class MockBuilder implements Builder {
    @override
    Future build(BuildStep buildStep) async {
      final entryLib = await buildStep.inputLibrary;
      if (entryLib == null) return;
      final sourceLibIsNonNullable = entryLib.isNonNullableByDefault;
      final mockLibraryAsset = buildStep.inputId.changeExtension('.mocks.dart');
      final objectsToMock = <DartObject>{};
  
      for (final element in entryLib.topLevelElements) {
  
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.');
  