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

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

Index: pkg/polymer/lib/src/build/polyfill_injector.dart
diff --git a/pkg/polymer/lib/src/build/polyfill_injector.dart b/pkg/polymer/lib/src/build/polyfill_injector.dart
index fefc14821ddb932b6a9e173d5f53b92311318ca0..f52947c9ac4c5331fd02451f50871a15521fafb0 100644
--- a/pkg/polymer/lib/src/build/polyfill_injector.dart
+++ b/pkg/polymer/lib/src/build/polyfill_injector.dart
@@ -112,6 +112,8 @@ class PolyfillInjector extends Transformer with PolymerTransformer {
           new Asset.fromString(transform.primaryInput.id, document.outerHtml));
     });
   }
+
+  String toString() => 'polymer-polyfill-injector';
 }
 
 final _shadowDomJS = new RegExp(r'shadow_dom\..*\.js', caseSensitive: false);