Disallow cyclic mixins.

sdk/lib/_internal/compiler/implementation/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
@@ skipping 294 matching lines @@
 
   static const ILLEGAL_MIXIN_APPLICATION_MODIFIERS = const MessageKind(
       "Error: illegal mixin application modifiers: #{1}.");
 
   static const ILLEGAL_MIXIN_SUPERCLASS = const MessageKind(
       "Error: class used as mixin must have Object as superclass.");
 
   static const ILLEGAL_MIXIN_CONSTRUCTOR = const MessageKind(
       "Error: class used as mixin cannot have non-factory constructor.");
 
+  static const ILLEGAL_MIXIN_CYCLE = const MessageKind(
+      "Error: class used as mixin introduces mixin cycle: #{1} <-> #{2}.");
+
   static const PARAMETER_NAME_EXPECTED = const MessageKind(
       "Error: parameter name expected.");
 
   static const CANNOT_RESOLVE_GETTER = const MessageKind(
       'cannot resolve getter.');
 
   static const CANNOT_RESOLVE_SETTER = const MessageKind(
       'cannot resolve setter.');
 
   static const VOID_NOT_ALLOWED = const MessageKind(
@@ skipping 150 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }