anchor.diff:      }
     }
   }
  +
  +class MockNavigationLocationProviderWithFiles implements NavigationLocationProvider {
  +  bool first = true;
  +
  +  NavigationLocation currentMockLocation = null;
  +
  +  NavigationLocation get navigationLocation {
  +    if (first) {
  +      first = false;
  +      return null;
  +    } else {
  +      return currentMockLocation;
  +    }
  +  }
  +}
  
positive.code:       expect(manager.currentLocation, isNotNull);
        expect(manager.forwardLocation, isNotNull);
      });
    });
  }
  
  
negative.code: 
  import 'dart:async';
  
  import 'files_mock.dart';
  import 'package:unittest/unittest.dart';
  
  import 'ace_test.dart';
  import '../lib/preferences.dart';
  import '../lib/navigation.dart';
  import '../lib/workspace.dart';
  import '../lib/ace.dart';
  
  defineTests() {
    group('navigation', () {
  