Clean VM thread shutdown:

runtime/include/dart_api.h

 /*
  * Copyright (c) 2012, 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 INCLUDE_DART_API_H_
 #define INCLUDE_DART_API_H_
 
 /** \mainpage Dart Embedding API Reference
@@ skipping 350 matching lines @@
  */
 #define DART_CHECK_VALID(handle)                                               \
   {                                                                            \
     Dart_Handle __handle = handle;                                             \
     if (Dart_IsError((__handle))) {                                            \
       _Dart_ReportErrorHandle(__FILE__, __LINE__,                              \
                               #handle, Dart_GetError(__handle));               \
     }                                                                          \
   }                                                                            \
 
-
 /**
  * Converts an object to a string.
  *
  * May generate an unhandled exception error.
  *
  * \return The converted string if no error occurs during
  *   the conversion. If an error does occur, an error handle is
  *   returned.
  */
 DART_EXPORT Dart_Handle Dart_ToString(Dart_Handle object);
@@ skipping 472 matching lines @@
  *   or NULL if no snapshot is provided.
  * \param create A function to be called during isolate creation.
  *   See Dart_IsolateCreateCallback.
  * \param interrupt A function to be called when an isolate is interrupted.
  *   See Dart_IsolateInterruptCallback.
  * \param unhandled_exception A function to be called if an isolate has an
  *   unhandled exception.  Set Dart_IsolateUnhandledExceptionCallback.
  * \param shutdown A function to be called when an isolate is shutdown.
  *   See Dart_IsolateShutdownCallback.
  *
- * \return True if initialization is successful.
+ * \return NULL if initialization is successful. Returns an error message
+ *   otherwise. The caller is responsible for freeing the error message.
  */
-DART_EXPORT bool Dart_Initialize(
+DART_EXPORT char* Dart_Initialize(
     const uint8_t* vm_isolate_snapshot,
     Dart_IsolateCreateCallback create,
     Dart_IsolateInterruptCallback interrupt,
     Dart_IsolateUnhandledExceptionCallback unhandled_exception,
     Dart_IsolateShutdownCallback shutdown,
     Dart_FileOpenCallback file_open,
     Dart_FileReadCallback file_read,
     Dart_FileWriteCallback file_write,
     Dart_FileCloseCallback file_close,
     Dart_EntropySource entropy_source);
 
 /**
  * Cleanup state in the VM before process termination.
  *
- * \return True if cleanup is successful.
+ * \return NULL if cleanup is successful. Returns an error message otherwise.
+ *   The caller is responsible for freeing the error message.
  */
-DART_EXPORT bool Dart_Cleanup();
+DART_EXPORT char* Dart_Cleanup();
 
 /**
  * Sets command line flags. Should be called before Dart_Initialize.
  *
  * \param argc The length of the arguments array.
  * \param argv An array of arguments.
  *
  * \return True if VM flags set successfully.
  */
 DART_EXPORT bool Dart_SetVMFlags(int argc, const char** argv);
@@ skipping 1939 matching lines @@
 
 /**
  * Returns the port that script load requests should be sent on.
  *
  * \return Returns the port for load requests or ILLEGAL_PORT if the service
  * isolate failed to startup or does not support load requests.
  */
 DART_EXPORT Dart_Port Dart_ServiceWaitForLoadPort();
 
 #endif  /* INCLUDE_DART_API_H_ */  /* NOLINT */