anchor.diff:    /// Run the dart2js compiler.
     Future _doCompilation(Transform transform) {
       var provider = new _BarbackCompilerProvider(_environment, transform,
  -        generateSourceMaps: _settings.mode != BarbackMode.RELEASE);
  +        generateSourceMaps: _generateSourceMaps);
   
       // Create a "path" to the entrypoint script. The entrypoint may not actually
       // be on disk, but this gives dart2js a root to resolve relative paths
  
positive.code:     var primaryId = transform.primaryId;
      transform.declareOutput(primaryId.addExtension(".js"));
      transform.declareOutput(primaryId.addExtension(".precompiled.js"));
      if (generateSourceMaps) {
        transform.declareOutput(primaryId.addExtension(".js.map"));
      }
      return new Future.value();
  
negative.code: 
    final BuildEnvironment _environment;
    final BarbackSettings _settings;
  
    /// Whether source maps should be generated for the compiled JS.
    bool get _generateSourceMaps => _settings.mode != BarbackMode.RELEASE;
  
    Dart2JSTransformer.withSettings(this._environment, this._settings) {
      var invalidOptions = _settings.configuration.keys.toSet()
  