Ensure that descriptors have a prototype.

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

 // Copyright (c) 2013, 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;
 
 // TODO(ahe): Share these with js_helper.dart.
 const FUNCTION_INDEX = 0;
 const NAME_INDEX = 1;
 const CALL_NAME_INDEX = 2;
@@ skipping 37 matching lines @@
       emitter.generateEmbeddedGlobalAccess(embeddedNames.MANGLED_GLOBAL_NAMES);
   jsAst.Expression mangledNamesAccess =
       emitter.generateEmbeddedGlobalAccess(embeddedNames.MANGLED_NAMES);
   jsAst.Expression librariesAccess =
       emitter.generateEmbeddedGlobalAccess(embeddedNames.LIBRARIES);
   jsAst.Expression typesAccess =
       emitter.generateEmbeddedGlobalAccess(embeddedNames.TYPES);
 
   jsAst.Statement processClassData = js.statement('''{
   function processClassData(cls, descriptor, processedClasses) {
-    var newDesc = map(); // Use a slow object.
+    var newDesc = {};
+    newDesc.x = 0; delete newDesc.x; // Make object slow.
     var previousProperty;
     var properties = Object.keys(descriptor);
     for (var i = 0; i < properties.length; i++) {
       var property = properties[i];
       var firstChar = property.substring(0, 1);
       if (property === "static") {
         processStatics(#embeddedStatics[cls] = descriptor[property],
                        processedClasses);
       } else if (firstChar === "+") {
         mangledNames[previousProperty] = property.substring(1);
@@ skipping 415 matching lines @@
 (function() {
   var result = $array[$index];
   if ($check) {
     throw new Error(
         name + ": expected value of type \'$type\' at index " + ($index) +
         " but got " + (typeof result));
   }
   return result;
 })()''';
 }