Fix crash when posting a message to a send port with port id 0.

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 1156 matching lines @@
  *
  * \param port The destination port.
  * \param object An object from the current isolate.
  *
  * \return True if the message was posted.
  */
 DART_EXPORT bool Dart_Post(Dart_Port port_id, Dart_Handle object);
 
 /**
  * Returns a new SendPort with the provided port id.
+ *
+ * \param port_id The destination port.
+ *
+ * \return A new SendPort if no errors occurs. Otherwise returns
+ *   an error handle.
  */
 DART_EXPORT Dart_Handle Dart_NewSendPort(Dart_Port port_id);
 
 /**
  * Gets the SendPort id for the provided SendPort.
  * \param port A SendPort object whose id is desired.
  * \param port_id Returns the id of the SendPort.
  * \return Success if no error occurs. Otherwise returns
  *   an error handle.
  */
@@ skipping 1642 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 */