Fix http://dartbug.com/8669 by making sure we do not crash when trying to use Object as a mixin.

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 309 matching lines @@
 
   static const ILLEGAL_CONSTRUCTOR_MODIFIERS = const MessageKind(
       "Error: illegal constructor modifiers: #{modifiers}.");
 
   static const ILLEGAL_MIXIN_APPLICATION_MODIFIERS = const MessageKind(
       "Error: illegal mixin application modifiers: #{modifiers}.");
 
   static const ILLEGAL_MIXIN_SUPERCLASS = const MessageKind(
       "Error: class used as mixin must have Object as superclass.");
 
+  static const ILLEGAL_MIXIN_OBJECT = const MessageKind(
+      "Error: cannot use Object as mixin.");
+
   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: "
       "#{mixinName1} <-> #{mixinName2}.");
 
   static const ILLEGAL_MIXIN_WITH_SUPER = const MessageKind(
       "Error: cannot use class #{className} as a mixin because it uses super.");
 
@@ skipping 270 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }