Allow native classes to mixin behavior from an ordinary Dart class.

sdk/lib/_internal/compiler/implementation/native_handler.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.
 
 library native;
 
 import 'dart:collection' show Queue;
 import 'dart:uri';
 import 'dart2jslib.dart' hide SourceString;
 import 'elements/elements.dart';
@@ skipping 421 matching lines @@
     // be instantiated (abstract or simply unused).
     addSubtypes(cls.superclass, emitter);
 
     for (DartType type in cls.allSupertypes) {
       List<Element> subtypes = emitter.subtypes.putIfAbsent(
           type.element,
           () => <ClassElement>[]);
       subtypes.add(cls);
     }
 
+    // Skip through all the mixin applications in the super class
+    // chain. That way, the direct subtypes set only contain the
+    // natives classes.
+    ClassElement superclass = cls.superclass;
+    while (superclass != null && superclass.isMixinApplication) {
+      assert(!superclass.isNative());
+      superclass = superclass.superclass;
+    }
+
     List<Element> directSubtypes = emitter.directSubtypes.putIfAbsent(
-        cls.superclass,
+        superclass,
         () => <ClassElement>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(log(message)) {
     log('Compiled ${registeredClasses.length} native classes, '
         '${unusedClasses.length} native classes omitted.');
   }
 }
 
@@ skipping 423 matching lines @@
   } else {
     if (parameters.parameterCount != 0) {
       compiler.cancel(
           'native "..." syntax is restricted to functions with zero parameters',
           node: nativeBody);
     }
     LiteralString jsCode = nativeBody.asLiteralString();
     builder.push(new HForeign.statement(jsCode.dartString, <HInstruction>[]));
   }
 }