2.0-preview issue: paper-menu-button-animations.html doesn't import web-animations.html

[flashlee]

Description

In 1.x element`s version paper-menu-button-animations.html imports web-animations.html form neon-animation. (https://github.com/PolymerElements/paper-menu-button/blob/master/paper-menu-button-animations.html#L12)
Here is no such import in 2.0-preview version.

Expected outcome

When click on dropdown-trigger element dropdown-content should appear with animation.

Actual outcome

When click on dropdown-trigger element dropdown-content appears without any animation.
Also error is appeared in dev console:
Couldnt play ( fade-in-animation ). ReferenceError: KeyframeEffect is not defined

[rene-lindner-isw]

I think this is intended.
https://www.webcomponents.org/element/PolymerElements/paper-menu-button -> Changes in 2.0