Disallow mixing in classes where the superclass != Object.

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 288 matching lines @@
 
   static const ILLEGAL_FINAL_METHOD_MODIFIER = const MessageKind(
       "Error: cannot have final modifier on method.");
 
   static const ILLEGAL_CONSTRUCTOR_MODIFIERS = const MessageKind(
       "Error: illegal constructor modifiers: #{1}.");
 
   static const ILLEGAL_MIXIN_APPLICATION_MODIFIERS = const MessageKind(
       "Error: illegal mixin application modifiers: #{1}.");
 
+  static const ILLEGAL_MIXIN_SUPERCLASS = const MessageKind(
+      "Error: illegal superclass for class used as mixin.");

[Johnni Winther, 2013/01/22 08:57:50]

Maybe explain why the superclass is illegal, that is that it is not Object.

[kasperl, 2013/01/22 09:00:28]

Done.

+
   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);
 }