Adding quick hack for directory override.

lib/dart2js_task.dart

@@ -14,7 +14,7 @@ import 'package:logging/logging.dart';
 Logger get _logger => new Logger('dart2js');
 
 /** Adds a dart2js task to the build configuration. */
-Dart2JSTask dart2js({String name: "dart2js", List<String> files}) => 
+Dart2JSTask dart2js({String name: "dart2js", List<String> files}) =>
     addTask(files, new Dart2JSTask(name));
 
 Path get _dart2jsPath => new Path.fromNative(new Options().executable)
@@ -23,21 +23,28 @@ Path get _dart2jsPath => new Path.fromNative(new Options().executable)
 /** Runs dart2js on the input files. */
 class Dart2JSTask extends Task {
 
+  Path outDirectory;
+
   Dart2JSTask(String name) : super(name);
-
+  Dart2JSTask.withOutDir(String name, this.outDirectory) : super(name);
+
   Future<TaskResult> run(List<InputFile> files, Path outDir, Path genDir) {
+    if (outDirectory != null) {
+      outDir = outDirectory;
+    }
+
     _logger.info("dart2js task starting. files: $files");
     var futureGroup = new FutureGroup();
     for (var file in files) {
       var outPath = outDir.append('${file.path}.js');
       var outFileDir = outPath.directoryPath;
-      
+
       new Directory.fromPath(outFileDir).createSync(recursive: true);
-      
+
       var options = new ProcessOptions()
         ..workingDirectory = new Directory.current().path;
       var args = ['--out=$outPath', '--verbose', file.inputPath.toNativePath()];
-      
+
       _logger.fine("running $_dart2jsPath args: $args");
       futureGroup.add(Process.run(_dart2jsPath.toNativePath(), args, options)
         ..transform((ProcessResult result) {