Yesterday's deadlock fix was wrong. It could drop notifications.

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 1090 matching lines @@
  */
 DART_EXPORT Dart_Handle Dart_HandleMessage();
 
 /**
  * Handles any pending messages for the vm service for the current
  * isolate.
  *
  * This function may be used by an embedder at a breakpoint to avoid
  * pausing the vm service.
  *
+ * This function can indirectly cause the message notify callback to
+ * be called.
+ *
  * \return true if the vm service requests the program resume
  * execution, false otherwise
  */
 DART_EXPORT bool Dart_HandleServiceMessages();
 
 /**
  * Does the current isolate have pending service messages?
  *
  * \return true if the isolate has pending service messages, false otherwise.
  */
@@ skipping 1700 matching lines @@
  * NOTE: If multiple callbacks with the same name are registered, only the
  * last callback registered will be remembered.
  */
 DART_EXPORT void Dart_RegisterRootServiceRequestCallback(
     const char* name,
     Dart_ServiceRequestCallback callback,
     void* user_data);
 
 
 #endif  /* INCLUDE_DART_API_H_ */  /* NOLINT */