anchor.diff:  NavigationLocation _mockLocation() {
     return new NavigationLocation(null, new Span(navigationOffset++, 0));
   }
  +
  +NavigationLocation _mockLocationWithFile(File mockFile) {
  +  return new NavigationLocation(mockFile, new Span(navigationOffset++, 0));
  +}
  +
   
   class MockNavigationLocationProvider implements NavigationLocationProvider {
     bool first = true;
  
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', () {
  