Don't export unnamed mixins.

pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart

 // Copyright (c) 2016, 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 fasta.kernel_library_builder;
 
 import 'dart:convert' show JSON;
 
 import 'package:front_end/src/fasta/combinator.dart' as fasta;
 import 'package:front_end/src/fasta/export.dart';
@@ skipping 224 matching lines @@
     bool isNamed = name != null;
     SourceClassBuilder builder;
     if (isNamed) {
       modifiers |= namedMixinApplicationMask;
     } else {
       name = supertype.name;
       int index = name.indexOf("^");
       if (index != -1) {
         name = name.substring(0, index);
       }
-      name = "$name&${mixin.name}$signature";
+      name = "_$name&${mixin.name}$signature";
       builder = mixinApplicationClasses[name];
     }
     if (builder == null) {
       builder = new SourceClassBuilder(
           documentationComment,
           metadata,
           modifiers,
           name,
           typeVariables,
           supertype,
@@ skipping 698 matching lines @@
           mixinApplicationClasses.putIfAbsent(name, () => builder);
       if (existing != builder) {
         part.scope.local.remove(name);
       }
     });
     super.includePart(part);
     nativeMethods.addAll(part.nativeMethods);
     boundlessTypeVariables.addAll(part.boundlessTypeVariables);
   }
 }