WheelEvent.wheelDeltaX throws exception in dart2js

[DartBot]

This issue was originally filed by ferha...@google.com

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What steps will reproduce the problem?

1. Add event handler: div.onMouseWheel.listen
2. Access WheelEvent member wheelDeltaX or Y in handler.

What is the expected output? What do you see instead?
In Dartium we get trackpad/mousewheel scroll amount. Dart2js compile gives exception.

What version of the product are you using?
Dart SDK version 1.10.0-dev.1.1

On what operating system?
Mac OSX Yosemite 10.10.3
What browser (if applicable)?
Chrome Version 42.0.2311.82 beta (64-bit)

Please provide any additional information below.

[dgrove]

This has been brittle over the years - in addition to fixing this, we need to improve our testing here.

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Set owner to @alan-knight.
Removed Priority-Unassigned label.
Added Priority-High, Library-Html, Triaged labels.

[alan-knight]

Part of the brittleness here is that the underlying specs have changed. The wheelDeltaX attribute is part of the obsolete MouseWheelEvent interface https://developer.mozilla.org/en-US/docs/Web/API/MouseWheelEvent , which is deprecated in favour of WheelEvent https://developer.mozilla.org/en-US/docs/Web/API/WheelEvent

It is still present in the Chrome IDL, but listed as non-standard API, and doesn't work on Firefox. It should probably be either removed or deprecated.

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Added Accepted label.

[alan-knight]

Added Duplicate label.
Marked as being merged into #16046.