[polymer] fix import inliner to only rewrite URL attributes once per node

pkg/polymer/test/build/common.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.
 
 library polymer.test.build.common;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 105 matching lines @@
       // We only check messages when an expectation is provided.
       if (messages == null) return;
       expect(messages.length, messagesSeen,
           reason: 'less messages than expected');
     });
   }
 }
 
 testPhases(String testName, List<List<Transformer>> phases,
     Map<String, String> inputFiles, Map<String, String> expectedFiles,
-    [List<String> expectedMessages]) {
-  test(testName, () {
+    [List<String> expectedMessages, bool solo = false]) {
+  (solo ? solo_test : test)(testName, () {
     var helper = new TestHelper(phases, inputFiles, expectedMessages)..run();
     return helper.checkAll(expectedFiles).then((_) => helper.tearDown());
   });
 }
 
 const WEB_COMPONENTS_TAG =
     '<script src="packages/web_components/platform.js"></script>\n'
     '<script src="packages/web_components/dart_support.js"></script>\n';
 
 const INTEROP_TAG = '<script src="packages/browser/interop.js"></script>\n';
 const DART_JS_TAG = '<script src="packages/browser/dart.js"></script>';