Ensure that descriptors have a prototype.

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart

 // Copyright (c) 2014, 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 OldEmitter implements Emitter {
   final Compiler compiler;
   final CodeEmitterTask task;
@@ skipping 458 matching lines @@
         function() {
           function tmp() {}
           return function (constructor, superConstructor) {
             tmp.prototype = superConstructor.prototype;
             var object = new tmp();
             object.x = 0; delete object.x; // Make object slow.
             var properties = constructor.prototype;
             var members = Object.keys(properties);
             for (var i = 0; i < members.length; i++) {
               var member = members[i];
+              if (!Object.hasOwnProperty.call(properties, member)) {
+                continue;
+              }
               object[member] = properties[member];
             }
             // Use a function for `true` here, as functions are stored in the
             // hidden class and not as properties in the object.
             object[#operatorIsPrefix + constructor.name] = constructor;
             object.constructor = constructor;
             constructor.prototype = object;
             return object;
           };
         }()
@@ skipping 1532 matching lines @@
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
 
       }
     }
   }
 }