anchor.diff:  
     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()
  
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:   /// Run the dart2js compiler.
    Future _doCompilation(Transform transform) {
      var provider = new _BarbackCompilerProvider(_environment, transform,
          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
  