Support metadata on patches.

tests/compiler/dart2js/mirrors_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'package:compiler/src/mirrors/source_mirrors.dart';
 import 'package:compiler/src/mirrors/mirrors_util.dart';
 import 'package:compiler/src/mirrors/analyze.dart';
 import 'package:compiler/src/filenames.dart'
@@ skipping 165 matching lines @@
   var dartMirrorsLibrary = system.libraries[DART_MIRRORS_URI];
   Expect.isNotNull(dartMirrorsLibrary);
   var commentType = dartMirrorsLibrary.declarations[#Comment];
   Expect.isNotNull(commentType);
 
   // /// Singleline doc comment.
   metadata = metadataList[metadataListIndex++];
   Expect.isTrue(metadata is InstanceMirror);
   Expect.isFalse(metadata.hasReflectee);
   Expect.throws(() => metadata.reflectee, (_) => true);
-  Expect.isTrue(metadata is CommentInstanceMirror);
+  Expect.isTrue(metadata is CommentInstanceMirror,
+      "Unexpected metadata: $metadata");
   Expect.equals(commentType.originalDeclaration, metadata.type);
   Expect.isTrue(metadata.isDocComment);
   Expect.stringEquals(
       "/// Singleline doc comment.", metadata.text);
   Expect.stringEquals(
       "Singleline doc comment.", metadata.trimmedText);
 
   // @Metadata
   metadata = metadataList[metadataListIndex++];
   Expect.isTrue(metadata is InstanceMirror);
@@ skipping 789 matching lines @@
   Expect.isTrue(privateFactoryConstructor.isPrivate);
   Expect.isFalse(privateFactoryConstructor.isConstConstructor);
   Expect.isFalse(privateFactoryConstructor.isRedirectingConstructor);
   Expect.isFalse(privateFactoryConstructor.isGenerativeConstructor);
   Expect.isTrue(privateFactoryConstructor.isFactoryConstructor);
 
   var metadata = privateClass.metadata;
   Expect.isNotNull(metadata);
   Expect.equals(0, metadata.length);
 }