Make EventPluginUtils clear dispatchIDs and dispatchListeners

The other dispatch utils clear these out and I found a bug where they weren't being cleared out.
I'm thinking this should probably be done in the `SyntheticEvent` constructor but at least this way it's consistent. Also, I know no one else uses `executeDispatchesInOrderStopAtTrue` except the `ResponderEventPlugin`.
Merge pull request #1324 from jordwalke/executeDispatchesInOrder

Make EventPluginUtils clear dispatchIDs and dispatchListeners