Disallow mixing in classes where the superclass != Object.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

 // Copyright (c) 2012, 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 resolution;
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(Node node);
@@ skipping 479 matching lines @@
   }
 
   void checkClass(ClassElement element) {
     if (element.isMixinApplication) {
       checkMixinApplication(element);
     } else {
       checkClassMembers(element);
     }
   }
 
-  void checkMixinApplication(MixinApplicationElement mixin) {
-    Modifiers modifiers = mixin.modifiers;
+  void checkMixinApplication(MixinApplicationElement mixinApplication) {
+    Modifiers modifiers = mixinApplication.modifiers;
     int illegalFlags = modifiers.flags & ~Modifiers.FLAG_ABSTRACT;
     if (illegalFlags != 0) {
       Modifiers illegalModifiers = new Modifiers.withFlags(null, illegalFlags);
       CompilationError error =
           MessageKind.ILLEGAL_MIXIN_APPLICATION_MODIFIERS.error(
               [illegalModifiers]);
       compiler.reportMessage(compiler.spanFromSpannable(modifiers),
                              error, Diagnostic.ERROR);
     }
+
+    ClassElement mixin = mixinApplication.mixin;
+    if (!mixin.superclass.isObject(compiler)) {
+      CompilationError error = MessageKind.ILLEGAL_MIXIN_SUPERCLASS.error();
+      compiler.reportMessage(compiler.spanFromElement(mixin),
+                             error, Diagnostic.ERROR);
+    }
   }
 
   void checkClassMembers(ClassElement cls) {
     assert(invariant(cls, cls.isDeclaration));
     if (cls.isObject(compiler)) return;
     // TODO(johnniwinther): Should this be done on the implementation element as
     // well?
     cls.forEachMember((holder, member) {
       compiler.withCurrentElement(member, () {
         // Perform various checks as side effect of "computing" the type.
@@ skipping 2971 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }