Cannot set property _isFinished of #<Object> which has only a getter

[zh99998]

I got Cannot set property _isFinished of #<Object> which has only a getter
in https://github.com/web-animations/web-animations-js/blob/master/src/animation.js#L194

seems _isFinished is only a getter but cancel() trying to set value.

[ewilligers]

We'll need more information if we are to reproduce this.
For example, which browser version are you using?

[dstoc]

It might help, but if you look at the code it seems pretty obviously broken.

[zh99998]

yes it's obviously broken, not browser related, any browser without native Web Animation API could reproduce it

[dstoc]

@ewilligers this should repro with 'use strict'; enabled. It's probably safe to remove the line setting _isFinished, since it's a no-op otherwise.