Second event handler not correctly registered

It appears that on an element having more than one event handlers (on* attributes), the second one is not correctly registered but the first and third ones are. I haven't tried with more than 3.

**Test case:** https://codepen.io/stephanebisson/pen/yLrWgzL?editors=1010

Start by reproducing the reported behavior in the linked CodePen with elements that have multiple on* event-handler attributes, confirming that the second handler fails while the first and third work. Trace the event-handler registration path and add a regression test for multiple handlers; done means every handler is registered and invoked in order.

Written by the indexing model from the issue text.