Don't serialize unnamed mixin applications.

pkg/compiler/lib/src/serialization/serialization.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;
 
-import '../common/backend_api.dart';
 import '../elements/elements.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../universe/world_impact.dart';
 
 import 'constant_serialization.dart';
 import 'element_serialization.dart';
-import 'impact_serialization.dart';
 import 'json_serializer.dart';
 import 'keys.dart';
 import 'type_serialization.dart';
 import 'values.dart';
 
 /// An object that supports the encoding an [ObjectValue] for serialization.
 ///
 /// The [ObjectEncoder] ensures that nominality and circularities of
 /// non-primitive values like [Element], [DartType] and [ConstantExpression] are
 /// handled.
@@ skipping 883 matching lines @@
 
   /// Returns the value used to store [key] as a property in the encoding an
   /// [ObjectValue].
   ///
   /// Different encodings have different restrictions and capabilities as how
   /// to store a [Key] value. For instance: A JSON encoding needs to convert
   /// [Key] to a [String] to store it in a JSON object; a Dart encoding can
   /// choose to store a [Key] as an [int] or as the [Key] itself.
   getObjectPropertyValue(Key key);
 }