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

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 6feb5eaa2830e65b923da92b9520ca4b2bde36eb..af5f97797c174d925f6e1e89cb09d4e823b05017 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -1174,6 +1174,11 @@ 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);