iron-collapse tests failing due to missed transitionend event

[jakemac53]

The event handler in the element itself is firing (_onTransitionEnd) but imperative listeners aren't getting called so something weird is going on there.

[ghost]

As you probably know by now, changing collapse.on['transitionend'] to collapse.onTransitionEnd can pass the test and works fine on Dartium, while on Chrome listeners aren't still getting called in the test and actual code, I believe.

I find following code in my sample project (index.bootstrap.initialize.dart.js after pub build).

{if(P.fR())return"webkitTransitionEnd"
else if(P.dh())return"oTransitionEnd"
return"transitionend"}

When I remove the webkitTransitionEnd and oTransitionEnd flow to get always returned transitionend, then the listener is getting called. So maybe there is a dart2js's bug?