Add missing locking around loaders list

runtime/bin/loader.h

 // Copyright (c) 2016, 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.
 
 #ifndef BIN_LOADER_H_
 #define BIN_LOADER_H_
 
 #include "bin/isolate_data.h"
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
@@ skipping 98 matching lines @@
   // The map of active loaders.
   static Mutex loader_infos_lock_;
   static LoaderInfo* loader_infos_;
   static intptr_t loader_infos_length_;
   static intptr_t loader_infos_capacity_;
 
   static void AddLoader(Dart_Port port, IsolateData* data);
   static void RemoveLoader(Dart_Port port);
   static intptr_t LoaderIndexFor(Dart_Port port);
   static Loader* LoaderFor(Dart_Port port);
+  static Loader* LoaderForLocked(Dart_Port port);
 
   // This is the global callback for the native message handlers.
   static void NativeMessageHandler(Dart_Port dest_port_id,
                                    Dart_CObject* message);
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_LOADER_H_