Update tonic for VM roll

dart_vm.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TONIC_DART_VM_H_
 #define TONIC_DART_VM_H_
 
 #include <vector>
 
 #include "dart/runtime/include/dart_api.h"
 
 namespace tonic {
 
 // Initialize and Cleanup the Dart VM.
 class DartVM {
  public:
   struct Config {
     Config();
 
     const uint8_t* vm_isolate_snapshot;
     const uint8_t* instructions_snapshot;
     const uint8_t* data_snapshot;
     Dart_IsolateCreateCallback create;
     Dart_IsolateInterruptCallback interrupt;
     Dart_IsolateUnhandledExceptionCallback unhandled_exception;
     Dart_IsolateShutdownCallback shutdown;
+    Dart_ThreadExitCallback thread_exit;
     Dart_FileOpenCallback file_open;
     Dart_FileReadCallback file_read;
     Dart_FileWriteCallback file_write;
     Dart_FileCloseCallback file_close;
     Dart_EntropySource entropy_source;
     Dart_GetVMServiceAssetsArchive get_service_assets;
   };
 
   // Initialize the Dart VM using |config| and |flags|. Returns |false| on
   // error and then the error message can be retrieved by calling |error|.
   static bool Initialize(const Config& config,
                          const std::vector<const char*>& flags);
 
   // Cleanup the Dart VM. Returns |false| on error and then the error message
   // can be retrieved by calling |error|.
   static bool Cleanup();
 
   // Returns the error string after a failing call to |Cleanup| or |Initialize|.
   static const char* error();
 };
 
 }  // namespace tonic
 
 #endif  // TONIC_DART_VM_H_