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: 
  import 'dart:async';
  
  import 'package:unittest/unittest.dart';
  
  import '../lib/navigation.dart';
  
  defineTests() {
    group('navigation', () {
  
negative.code:     }
    }
  }
  
  class MockNavigationLocationProviderWithFiles implements NavigationLocationProvider {
    bool first = true;
  
    NavigationLocation currentMockLocation = null;
  
    NavigationLocation get navigationLocation {
      if (first) {
        first = false;
        return null;
      } else {
        return currentMockLocation;
      }
    }
  }
  