web-animations-js compatibility issue (AnimationPlaybackEvent TypeError)

[petearvanitis]

Description

A new release of web-animations-js came out on April 13th, 2017 (2.2.3). A compatibility issue with this release is causing the following error to be thrown whenever paper-dropdown-menu is clicked:

Uncaught TypeError: Cannot assign to read only property 'target' of object '#<AnimationPlaybackEvent>'
at b.Animation.<anonymous> (web-animations-next-animation.js:223)

Hard-coding v2.2.2 of web-animations-js in the bower file releases the issue. I'm witnessing this behavior on the 1.5.0 version of paper-dropdown-menu but I'm seeing it reported on the 2.0-preview version as well.

Expected outcome

Dropdown menu should draw the list items as a popup

Actual outcome

Works the first time; however, subsequent clicks not render the pop-up or renders a partially drawn popup. The following error is logged in the console:

Uncaught TypeError: Cannot assign to read only property 'target' of object '#<AnimationPlaybackEvent>'
at b.Animation.<anonymous> (web-animations-next-animation.js:223)

Live Demo

Broken Version (web-animations-js#2.2.3)
https://jsbin.com/jokilivehi/edit?html,output

Working Version(web-animations-js#2.2.2)
https://jsbin.com/wehojihamo/edit?html,output

Steps to reproduce

Add a sample paper-dropdown-menu to a page.
Update bower.
Verify v2.2.3 of web-animations-js is installed

Browsers Affected

• [X ] Chrome
• [X ] Firefox

[petearvanitis]

web-animations-js has released a 2.2.4 patch to fix this issue.

#web-animations/web-animations-js#124