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

pkg/polymer/lib/src/build/remove_sourcemap_comment.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 removes any sourcemap comments from javascript files.
-library web_components.src.remove_sourcemap_comment;
-
-import 'dart:async';
-import 'package:barback/barback.dart';
-
-/// Transformer that removes any sourcemap comments from javascript files.
-/// Comments should be on their own line in the form: //# sourceMappingURL=*.map
-class RemoveSourcemapComment extends Transformer {
-  BarbackSettings settings;
-
-  RemoveSourcemapComment.asPlugin(this.settings);
-
-  /// Only apply to files in release mode.
-  isPrimary(_) => settings.mode == BarbackMode.RELEASE;
-
-  apply(Transform transform) {
-    var id = transform.primaryInput.id;
-    return transform.readInputAsString(id).then((file) {
-      if (file.contains(_SOURCE_MAP_COMMENT)) {
-        transform.addOutput(new Asset.fromString(
-            id, file.replaceAll(_SOURCE_MAP_COMMENT, '')));
-      }
-    });
-  }
-}
-
-final RegExp _SOURCE_MAP_COMMENT = new RegExp(r'\n\s*\/\/# sourceMappingURL.*');