anchor.diff:    //TODO : several compilers ?
     var compiler = new DirectoryCompiler(directory, modules, fileListFactory,
         srcName:'web.src', outName:'web');
  -  compiler.start().then((_){
  +  
  +  return compiler.start().then((_){
     
       if(opts.clean) compiler.clean();
       
  
positive.code: import 'package:path/path.dart' as Path;
  
  
  build(List<String> args, [FileListFactory fileListFactory]){
    final opts = BuildOptions.parse(args);
    
    if (fileListFactory == null) {
  
negative.code:         .then((_) => Future.wait(opts.removed.map(map('web.src',compiler.removeFile))));
      }
      
      return _done.then((_){
        Iterable files = compiler.fileList.files.values;
        
        final result = new BuildResult();
  