RangeInputElement mouseup-event doesn't always provide the mouse position

[zoechi]

https://github.com/bwu-dart/polymer_elements_demos/blob/d67f9a4e9d35dc9b1e4c7f1d5a1185da2e91dad0/web/google_castable_video/google_castable_video_demo.dart#L49-L49

When I drag the knob and release it, the mouseup event doesn't always provide a position.
In the demo linked above this usually happens when I move the know to an advanced position and then move it back nearly to the beginning of the video.

[jakemac53]

Weird, can you repro this outside of the demo as well? (I can't seem to)

[jakemac53]

Since this is just a normal element it seems like it would be a general dartium bug, but if it only repros inside that demo then maybe something else is going on.

[zoechi]

Sorry, I added only here bwu-dart/polymer_elements_demos#8, that I can repro in Chrome (Linux) as well.