Also intialize methods inherited from mixin classes.

tests/compiler/dart2js_extra/23056_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.
+
+// Regression test for http://dartbug.com/23056/
+// Ensure that Mixin prototypes are initialized before first use.
+
+// Mirrors is the only way to have a getter be equipped with metadata.
+@MirrorsUsed(targets: 'M', override: '*')
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+class M {
+  @NoInline()
+  bool get foo => true;
+}
+
+class A extends Object with M {
+  @NoInline()
+  bool get foo => super.foo;
+}
+
+@AssumeDynamic() @NoInline()
+bool hide(bool x) => x;
+
+main() {
+  Expect.isTrue((hide(true) ? new A() : new M()).foo);
+}