Improve equivalence testing of unnamed mixin applications.

pkg/compiler/lib/src/common/codegen.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.
 
 library dart2js.common.codegen;
 
 import '../closure.dart' show SynthesizedCallMethodElementX;
 import '../common.dart';
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart' show ConstantValue;
@@ skipping 233 matching lines @@
       ResolvedAst resolvedAst, ItemCompilationContext compilationContext)
       : this.resolvedAst = resolvedAst,
         super(resolvedAst.element, compilationContext);
 
   WorldImpact run(Compiler compiler, CodegenEnqueuer world) {
     if (world.isProcessed(element)) return const WorldImpact();
 
     registry = new CodegenRegistry(compiler, element);
     return compiler.codegen(this, world);
   }
+
+  String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }