Fix BaseEvent inheritance

[c9s]

This PR fixes the following errors:

(237,36): error TS2339: Property 'layerX' does not exist on type 'Event | BaseEvent'.
(236,56): error TS2339: Property 'layerY' does not exist on type 'Event | BaseEvent'.
(8,14): error TS2339: Property 'stopPropagation' does not exist on type 'Event | BaseEvent'.

[c9s]

For people who are looking for a workaround, here is the solution:

var e = <any>d3.event;

[blakeembrey]

I'm not sure BaseEvent is meant to extend Event. Do you have docs for this change?

The Event is the browser default event, while I believed d3 created it's own events with a sourceEvent pointing to the original event. If you know the event is an Event at that point in time, you should be using TypeScript for coercion - not changing the definition.

[c9s]

d3.event is a DOM Event. the following definition is from the d3 document:

 The d3.event object is a DOM event and implements the standard event fields like timeStamp and keyCode as well as methods like preventDefault() and stopPropagation().

[blakeembrey]

Cool, thanks. Did you have a link to that? Should d3.event be updated to be event: BaseEvent instead of a union of types?

[doggy8088]

@blakeembrey https://github.com/d3/d3/wiki/Selections