Improve equivalence testing of unnamed mixin applications.

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 202 matching lines @@
     bool found = false;
     for (ClassHierarchyNode other in node2.directSubclasses) {
       if (areElementsEquivalent(child.cls, other.cls)) {
         checkClassHierarchyNodes(compiler1, compiler2,
             child, other, verbose: verbose);
         found = true;
         break;
       }
     }
     if (!found) {
+      if (child.isInstantiated) {
+        print('Missing subclass ${child.cls} of ${node1.cls} '
+            'in ${node2.directSubclasses}');
+        print(compiler1.world.dump(
+            verbose ? compiler1.coreClasses.objectClass : node1.cls));
+        print(compiler2.world.dump(
+            verbose ? compiler2.coreClasses.objectClass : node2.cls));
+      }
       Expect.isFalse(child.isInstantiated,
-          'Missing subclass ${child.cls} of ${node1.cls}');
+          'Missing subclass ${child.cls} of ${node1.cls} in '
+              '${node2.directSubclasses}');
     }
   }
   checkMixinUses(compiler1, compiler2, node1.cls, node2.cls, verbose: verbose);
 }
 
 void checkSets(
     Iterable set1,
     Iterable set2,
     String messagePrefix,
     bool sameElement(a, b),
@@ skipping 155 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';
 }