Add missing Loader::InitOnce to gen_snapshot

runtime/bin/gen_snapshot.cc

 // 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.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ skipping 1186 matching lines @@
   // Parse command line arguments.
   if (ParseArguments(argc,
                      argv,
                      &vm_options,
                      &app_script_name) < 0) {
     PrintUsage();
     return 255;
   }
 
   Thread::InitOnce();
+  Loader::InitOnce();
   DartUtils::SetOriginalWorkingDirectory();
   // Start event handler.
   TimerUtils::InitOnce();
   EventHandler::Start();
 
 #if !defined(PRODUCT)
   // Constant true in PRODUCT mode.
   vm_options.AddArgument("--load_deferred_eagerly");
 #endif
 
@@ skipping 136 matching lines @@
   EventHandler::Stop();
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }