Add linter by default for polymer's pub-build, also cleans up the linter code.

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

 // Copyright (c) 2013, 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.
 
 /** Includes any additional polyfills that may needed by the deployed app. */
 library polymer.src.build.polyfill_injector;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 94 matching lines @@
         _addScript('custom_element/custom-elements$suffix');
       }
 
       // This polyfill needs to be the first one on the head
       if (!shadowDomFound) _addScript('shadow_dom/shadow_dom$suffix');
 
       transform.addOutput(
           new Asset.fromString(transform.primaryInput.id, document.outerHtml));
     });
   }
+
+  String toString() => 'polymer-polyfill-injector';
 }
 
 final _shadowDomJS = new RegExp(r'shadow_dom\..*\.js', caseSensitive: false);
 final _customElementJS = new RegExp(r'custom-elements\..*\.js',
     caseSensitive: false);