Revert VM thread cleanup

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index 5b93b5f7ec1b94e5724b355a4bb03678a08627aa..d3f54b1b48ede03ca59aad692b1cbabd5994f12d 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -861,10 +861,9 @@ typedef bool (*Dart_EntropySource)(uint8_t* buffer, intptr_t length);
  * \param shutdown A function to be called when an isolate is shutdown.
  *   See Dart_IsolateShutdownCallback.
  *
- * \return NULL if initialization is successful. Returns an error message
- *   otherwise. The caller is responsible for freeing the error message.
+ * \return True if initialization is successful.
  */
-DART_EXPORT char* Dart_Initialize(
+DART_EXPORT bool Dart_Initialize(
     const uint8_t* vm_isolate_snapshot,
     Dart_IsolateCreateCallback create,
     Dart_IsolateInterruptCallback interrupt,
@@ -879,10 +878,9 @@ DART_EXPORT char* Dart_Initialize(
 /**
  * Cleanup state in the VM before process termination.
  *
- * \return NULL if cleanup is successful. Returns an error message otherwise.
- *   The caller is responsible for freeing the error message.
+ * \return True if cleanup is successful.
  */
-DART_EXPORT char* Dart_Cleanup();
+DART_EXPORT bool Dart_Cleanup();
 
 /**
  * Sets command line flags. Should be called before Dart_Initialize.