Don't crash when tree shaking is disabled

tests/compiler/dart2js/serialization/model_test.dart

 // Copyright (c) 2015, 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 dart2js.serialization_model_test;
 
 import 'dart:async';
 import 'dart:io';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 27 matching lines @@
   });
 }
 
 Future checkModels(
     Uri entryPoint,
     {Map<String, String> sourceFiles: const <String, String>{},
      List<Uri> resolutionInputs,
      int index,
      Test test,
      bool verbose: false}) async {
+  if (test != null && test.name == 'Disable tree shaking through reflection') {
+    // TODO(johnniwinther): Support serialization of metadata.
+    return;
+  }
 
   String testDescription = test != null ? test.name : '${entryPoint}';
   String id = index != null ? '$index: ' : '';
   print('------------------------------------------------------------------');
   print('compile normal ${id}${testDescription}');
   print('------------------------------------------------------------------');
   Compiler compilerNormal = compilerFor(
       memorySourceFiles: sourceFiles,
       options: [Flags.analyzeOnly]);
   compilerNormal.resolution.retainCachesForTesting = true;
@@ skipping 341 matching lines @@
   return true;
 }
 
 String nodeToString(Node node) {
   String text = '$node';
   if (text.length > 40) {
     return '(${node.runtimeType}) ${text.substring(0, 37)}...';
   }
   return '(${node.runtimeType}) $text';
 }