anchor.diff:  
   import 'dart:async';
   import 'dart:io' show Directory, File, IOSink, Platform;
  -
  -import 'dart:typed_data' show Uint8List;
   
   import 'package:_fe_analyzer_shared/src/util/relativize.dart'
       show isWindows, relativizeUri;
  
positive.code:     }
      return new Future.value(pass(uri));
    }
  }
  
  class BytesCollector implements Sink<List<int>> {
    final List<List<int>> lists = <List<int>>[];
  
    int length = 0;
  
    @override
    void add(List<int> data) {
      lists.add(data);
      length += data.length;
    }
  
    Uint8List collect() {
      Uint8List result = new Uint8List(length);
      int offset = 0;
      for (List<int> list in lists) {
        result.setRange(offset, offset += list.length, list);
      }
      lists.clear();
      length = 0;
      return result;
    }
  
    @override
    void close() {}
  }
  
  Future<String> runDiff(Uri expected, String actual) async {
  
negative.code:         writeToFile = false;
        }
      }
      ByteSink sink = new ByteSink();
      bool good = false;
      try {
        // TODO(johnniwinther,jensj): Avoid serializing the sdk.
        new BinaryPrinter(sink).writeComponentFile(component);
        good = true;
      } catch (e, s) {
        return fail(result, e, s);
      } finally {
        if (good && writeToFile && !skipVm) {
          Directory tmp = await Directory.systemTemp.createTemp();
          Uri uri = tmp.uri.resolve("generated.dill");
          File generated = new File.fromUri(uri);
          IOSink ioSink = generated.openWrite();
          ioSink.add(sink.builder.takeBytes());
          await ioSink.close();
          result = new ComponentResult(
              result.description,
              result.component,
              result.userLibraries,
              result.compilationSetup,
              result.sourceTarget,
              uri);
          print("Wrote component to `${generated.path}`.");
        } else {
          print("Wrote component to memory.");
        }
      }
      return pass(result);
    }
  }
  
  class ReadDill extends Step<Uri, Uri, ChainContext> {
  