R1:7ab26aef662a

Internet Explorer compatibility fixes

Summary:
A few fixes for IE that makes this work in IE9+ and at least you can drag things and have it fail horribly in IE8 now. Ref T5023.

- try / catch around getBoundingClientRect because IE throws if the object is not attached to the DOM. The correct value can be inferred in these cases as 0,0
- turns out IE has different values for button here (1,2,4) than other browsers. special case this by using the other IE fact that 'which' will be null

Test Plan: dragged…