Fix length computation on init.mangledNames.

sdk/lib/_internal/compiler/js_lib/js_names.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.
 
 library dart._js_names;
 
 import 'dart:_js_embedded_names' show
     JsGetName,
     MANGLED_GLOBAL_NAMES,
     MANGLED_NAMES;
@@ skipping 112 matching lines @@
       if (_isInstance && key.startsWith(getterPrefix)) {
         int getterPrefixLength = getterPrefix.length;
         String setterPrefix = JS_GET_NAME(JsGetName.SETTER_PREFIX);
         result['$reflectiveName='] =
             '$setterPrefix${key.substring(getterPrefixLength)}';
       }
     }
     return result;
   }
 
-  int get _jsMangledNamesLength => JS('int', '#.length', _jsMangledNames);
+  int get _jsMangledNamesLength => JS('int', 'Object.keys(#).length',
+      _jsMangledNames);
 
   String operator[](String key) {
     if (_cache == null || _jsMangledNamesLength != _cacheLength) {
       _cache = _updateReflectiveNames();
       _cacheLength = _jsMangledNamesLength;
     }
     return _cache[key];
   }
 }
 
@@ skipping 19 matching lines @@
   return JsCache.fetch(names, name);
 }
 
 String unmangleAllIdentifiersIfPreservedAnyways(String str) {
   return JS("String",
             r"(#).replace(/[^<,> ]+/g,"
             r"function(m) { return #[m] || m; })",
             str,
             JS_EMBEDDED_GLOBAL('', MANGLED_GLOBAL_NAMES));
 }