Only run [Transformer.isPrimary] once for each asset/transformer pair.

pkg/polymer/lib/src/build/mirrors_remover.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 uses of mirrors from the polymer runtime, so that
 /// deployed applications are thin and small.
 library polymer.src.build.mirrors_remover;
 
 import 'dart:async';
 import 'package:barback/barback.dart';
 
 /// Removes the code-initialization logic based on mirrors.
 class MirrorsRemover extends Transformer {
   MirrorsRemover.asPlugin();
 
   /// Only apply to `lib/polymer.dart`.
-  Future<bool> isPrimary(Asset input) => new Future.value(
-      input.id.package == 'polymer' &&
-      input.id.path == 'lib/polymer.dart');
+  Future<bool> isPrimary(AssetId id) => new Future.value(
+      id.package == 'polymer' && id.path == 'lib/polymer.dart');
 
   Future apply(Transform transform) {
     var id = transform.primaryInput.id;
     return transform.primaryInput.readAsString().then((code) {
       // Note: this rewrite is highly-coupled with how polymer.dart is
       // written. Make sure both are updated in sync.
       var start = code.indexOf('@MirrorsUsed(');
       if (start == -1) _error();
       var end = code.indexOf('show MirrorsUsed;', start);
       if (end == -1) _error();
       end = code.indexOf('\n', end);
       var sb = new StringBuffer()
           ..write(code.substring(0, start))
           ..write(code.substring(end));
 
       transform.addOutput(new Asset.fromString(id, sb.toString()));
     });
   }
 }
 
 /** Transformer phases which should be applied to the smoke package. */
 List<List<Transformer>> get phasesForSmoke =>
     [[new MirrorsRemover.asPlugin()]];
 
 _error() => throw new StateError("Couldn't remove imports to mirrors, maybe "
     "polymer.dart was modified, but mirrors_remover.dart wasn't.");