Work around Safari for-in bug.

dart/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 62 matching lines @@
   preserveNames();
   var result = <String, String>{};
   map.forEach((String mangledName, String reflectiveName) {
     result[reflectiveName] = mangledName;
   });
   return result;
 }
 
 @NoInline()
 List extractKeys(victim) {
-  var result = JS('', '''
-(function(victim, hasOwnProperty) {
-  var result = [];
-  for (var key in victim) {
-    if (hasOwnProperty.call(victim, key)) result.push(key);
-  }
-  return result;
-})(#, Object.prototype.hasOwnProperty)''', victim);
+  var result = JS('', '# ? Object.keys(#) : []', victim, victim);
   return new JSArray.markFixed(result);
 }
 
 /**
  * Returns the (global) unmangled version of [name].
  *
  * Normally, you should use [mangledGlobalNames] directly, but this method
  * doesn't tell the compiler to preserve names. So this method only returns a
  * non-null value if some other component has made the compiler preserve names.
  *
  * This is used, for example, to return unmangled names from TypeImpl.toString
  * *if* names are being preserved for other reasons (use of dart:mirrors, for
  * example).
  */
 String unmangleGlobalNameIfPreservedAnyways(String name) {
   var names = JS_EMBEDDED_GLOBAL('=Object', MANGLED_GLOBAL_NAMES);
   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));
 }