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

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

Index: pkg/polymer/lib/src/build/script_compactor.dart
diff --git a/pkg/polymer/lib/src/build/script_compactor.dart b/pkg/polymer/lib/src/build/script_compactor.dart
index e869a2cb55c253bf7ffcc6f26bde5078241dcec8..f0b2315aa8acc911b4bbe52d0ac598285c6cf165 100644
--- a/pkg/polymer/lib/src/build/script_compactor.dart
+++ b/pkg/polymer/lib/src/build/script_compactor.dart
@@ -104,6 +104,8 @@ class ScriptCompactor extends Transformer with PolymerTransformer {
       });
     });
   }
+
+  String toString() => 'polymer-script-compactor';
 }
 
 const MAIN_HEADER = """