Deal better with missing sources in code_transformers and polymer

pkg/code_transformers/test/resolver_test.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.
 
 library code_transformers.test.resolver_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:code_transformers/resolver.dart';
@@ skipping 109 matching lines @@
 
     test('handles missing files', () {
       return validateResolver(
           inputs: {
             'a|web/main.dart': '''
               import 'package:b/missing.dart';
 
               main() {
               } ''',
           },
+          messages: [
+            'warning: Asset not found: b|lib/missing.dart',
+          ],
           validator: (resolver) {
             var lib = resolver.getLibrary(entryPoint);
             expect(lib.importedLibraries.length, 1);
           });
     });
 
     test('should update on changed package imports', () {
       return validateResolver(
           inputs: {
             'a|web/main.dart': '''
@@ skipping 216 matching lines @@
 
   // TODO(nweiz): This should just take an AssetId when barback <0.13.0 support
   // is dropped.
   Future<bool> isPrimary(idOrAsset) {
     var id = idOrAsset is AssetId ? idOrAsset : idOrAsset.id;
     return new Future.value(id == primary);
   }
 
   applyResolver(Transform transform, Resolver resolver) => validator(resolver);
 }