dart2js: Fix bad is-check in native_emitter.

pkg/compiler/lib/src/js_emitter/native_emitter.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 dart2js.js_emitter;
 
 class NativeEmitter {
 
   final Map<Element, ClassBuilder> cachedBuilders;
 
@@ skipping 314 matching lines @@
     bool needsAccessor(Field field) {
       return field.needsGetter ||
           field.needsUncheckedSetter ||
           field.needsCheckedSetter;
     }
 
     builder.isTrivial =
         cls.methods.isEmpty &&
         cls.isChecks.isEmpty &&
         cls.callStubs.isEmpty &&
-        superclass is! MixinApplicationElement &&
+        !superclass.isMixinApplication &&
         !cls.fields.any(needsAccessor);
 
     return builder;
   }
 
   void finishGenerateNativeClasses() {
     // TODO(sra): Put specialized version of getNativeMethods on
     // `Object.prototype` to avoid checking in `getInterceptor` and
     // specializations.
   }
@@ skipping 141 matching lines @@
         targetOutput.add(';');
       }
       targetOutput.addBuffer(jsAst.prettyPrint(
           new jsAst.ExpressionStatement(init), compiler));
       targetOutput.add('\n');
     }
 
     targetOutput.add('\n');
   }
 }