MouseEvent.getModifierState is unimplemented. KeyboardEvent.getModifierState is implemented; but the action on the MouseEvent isn't. We need to be consistent in our APIs this is a simple change to bring matching spec behavior. http://www.w3.org/TR/DOM-Level-3-Events/#widl-MouseEvent-getModifierState
Docs: https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/getModifierState
Samples: https://googlechrome.github.io/samples/mouseevent-get-modifier-state/index.html