Support for Gamepad API in dart:html

[DartBot]

This issue was originally filed by @pjako

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Gamepad API is around since some time now, can we get a dart integration for it?

https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html
https://developer.mozilla.org/en/API/Gamepad/Using_Gamepad_API

[dgrove]

Added Area-DOM, Triaged labels.

[vsmenon]

Added Duplicate label.
Marked as being merged into #1961.