use shared code transformer, update web_components to remove .map and .concat files

pkg/polymer/lib/src/build/delete_file.dart

-// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/// Transformer that deletes everything that it sees, but only in release mode.
-library web_components.src.delete_file;
-
-import 'dart:async';
-import 'package:barback/barback.dart';
-
-// Deletes all files supplied in release mode.
-class DeleteFile extends Transformer {
-  BarbackSettings settings;
-
-  DeleteFile.asPlugin(this.settings);
-
-  /// Only apply to files in release mode.
-  isPrimary(_) => settings.mode == BarbackMode.RELEASE;
-
-  apply(Transform transform) {
-    transform.consumePrimary();
-  }
-}