`transition-timing-function` property throws error on Safari & Firefox.

[cayter]

The error I got:

[Error] [object Object]
    dispatchOriginalEvent (webcomponents.js, line 957)
    dispatchEvent ([native code], line 0)
    dispatchEvent (webcomponents.js, line 1345)
    up (polymer.js, line 1987)
    gestureTrigger (polymer.js, line 813)
    (anonymous function) ([native code], line 0)
    (anonymous function) (web-animations-next-lite.min.js, line 15)
    forEach ([native code], line 0)
    c (web-animations-next-lite.min.js, line 15)

My animation definition:

<core-animation id="flip-in-x" target="{{ target }}" duration="{{ duration }}" easing="{{ easing }}" delay="{{ delay }}" endDelay="{{ endDelay }}" fill="{{ fill }}" iterations="{{ iterations }}" direction="{{ direction }}">
  <core-animation-keyframe offset="0">
    <core-animation-prop name="opacity" value="0"></core-animation-prop>
    <core-animation-prop name="transform" value="perspective(400px) rotate3d(1, 0, 0, 90deg)"></core-animation-prop>
    <core-animation-prop name="transition-timing-function" value="ease-in"></core-animation-prop>
  </core-animation-keyframe>
  <core-animation-keyframe offset="0.4">
    <core-animation-prop name="transform" value="perspective(400px) rotate3d(1, 0, 0, -20deg)"></core-animation-prop>
    <core-animation-prop name="transition-timing-function" value="ease-in"></core-animation-prop>
  </core-animation-keyframe>
  <core-animation-keyframe offset="0.6">
    <core-animation-prop name="opacity" value="1"></core-animation-prop>
    <core-animation-prop name="transform" value="perspective(400px) rotate3d(1, 0, 0, 10deg)"></core-animation-prop>
  </core-animation-keyframe>
  <core-animation-keyframe offset="0.8">
    <core-animation-prop name="transform" value="perspective(400px) rotate3d(1, 0, 0, -5deg)"></core-animation-prop>
  </core-animation-keyframe>
  <core-animation-keyframe offset="1">
    <core-animation-prop name="opacity" value="1"></core-animation-prop>
    <core-animation-prop name="transform" value="perspective(400px)"></core-animation-prop>
  </core-animation-keyframe>
</core-animation>

Thanks!