Actually it's 'kind of work', but not the way I wanted to. This is the result of setting custom easing:
Does it looks like there is no easing at all, right? Yes and no.
"Yes", there is no visual confirmation that animation has any easing, the object is moving with linear easing.
"No", if you open easing editor...
Yep, easing is there. When you press play button, it works. Once published - linear as usual.
What should I do to make Animate apply my custom easing???
So you have written a JSFL script that changes the easing for the selected frames?
Have you made sure to set hasCustomEase to true, for the frames?
Omg, I'm totally forgot about this param!
Yep, now it works!
For people like me, the right way to apply custom easing:
timeline.layers.frames.hasCustomEase = true;