Fix Dartium build:

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 <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 
 #include "include/dart_api.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 #include "bin/file.h"
 #include "bin/log.h"
+#include "bin/thread.h"
 
 #include "platform/globals.h"
 
 
 namespace dart {
 namespace bin {
 
 #define CHECK_RESULT(result)                                                   \
   if (Dart_IsError(result)) {                                                  \
     free(snapshot_buffer);                                                     \
@@ skipping 460 matching lines @@
 
   // Parse command line arguments.
   if (ParseArguments(argc,
                      argv,
                      &vm_options,
                      &app_script_name) < 0) {
     PrintUsage();
     return 255;
   }
 
+  Thread::InitOnce();
   DartUtils::SetOriginalWorkingDirectory();
 
   Dart_SetVMFlags(vm_options.count(), vm_options.arguments());
 
   // Initialize the Dart VM.
   // Note: We don't expect isolates to be created from dart code during
   // snapshot generation.
   if (!Dart_Initialize(NULL, NULL, NULL, NULL,
                        DartUtils::OpenFile,
                        DartUtils::ReadFile,
@@ skipping 73 matching lines @@
   }
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }