Don't export unnamed mixins.

pkg/front_end/test/mixin_export_test.dart

+// Copyright (c) 2017, 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:async_helper/async_helper.dart' show asyncTest;
+
+import 'package:front_end/src/testing/compiler_common.dart';
+import 'package:front_end/front_end.dart';
+
+main() {
+  asyncTest(() async {
+    var sources = <String, dynamic>{
+      'a.dart': 'class A extends Object with M {}  class M {}',
+      'b.dart': 'import "a.dart"; class B extends Object with M {}',
+      'c.dart': 'export "a.dart"; export "b.dart";',
+    };
+    await compileUnit(sources.keys.toList(), sources,
+        options: new CompilerOptions()
+          ..onError = (e) => throw '${e.severity}: ${e.message}');
+  });
+}