Polyfill event.dataTransfer.type for drag & drop events

[jansedivy]

This pull request demonstrates our idea (me and @hejld) how to fix #3656 and #2676. We started with just polyfilling the types property for now.

There are few aspects that are not finished yet - object pooling, checking if it is possible to use defineProperty, and tests - but it should be enough to gather initial feedback.

Thanks a lot!

cc @syranide

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[hejld]

@spicyj @zpao any thoughts on this? Should we proceed with implementation? Thanks!

[syranide]

Object.defineProperty isn't available on IE8.

Also, this.dataTransfer should be set using the same principle as in (or is there a reason not to?):
https://github.com/jansedivy/react/blob/drag-fix/src/renderers/dom/client/syntheticEvents/SyntheticMouseEvent.js#L34

It also seems problematic as this doesn't polyfill functionality universally and, as far as I can tell, will simply error if you try to use this API on a lesser browser. So it seems to me that this deviates from existing event polyfills provided by React and it's not immediately clear to me whether going this route is a good idea or not (long-term/maintainability/etc).

[facebook-github-bot]

@jansedivy updated the pull request.

[hejld]

Thanks! This helps a lot. Will get back to you once the feedback is incorporated.

[gaearon]

Thank you for taking time to contribute. I’m closing as this PR has not received updates in response to the feedback in #5529 (comment). Please feel free to open another one if you plan to get back to this!