https://treeherder.mozilla.org/#/jobs?repo=try&revision=b5800bb624450f22893e4918d653b3dbe231c4ac&selectedJob=147753921 It doesn't fail on local Android build though.

There is a horrible mistake that we call start() twice[1][2] to run all test cases! So actually we've been running whole test twice each run.:/ And the twice run interferes other running test cases somehow with the conformant Promise handling. [1] https://hg.mozilla.org/mozilla-central/file/f5f03ee9e6ab/dom/animation/test/chrome/test_animation_performance_warning.html#l37 [2] https://hg.mozilla.org/mozilla-central/file/f5f03ee9e6ab/dom/animation/test/chrome/test_animation_performance_warning.html#l1298 Here is a try with dropping the one call; https://treeherder.mozilla.org/#/jobs?repo=try&revision=e3822d7b0b4981ddfd0f3a3ec52690c5c0e5160f In this try, test_animation_performance_warning.html took 70805ms on Android opt build, whereas it took 134372ms on Android opt build without the fix. :)

Comment on attachment 8932351 [details] Bug 1421151 - Drop redundant calling start(). https://reviewboard.mozilla.org/r/203394/#review208834 Wow, nice find.

Pushed by hikezoe@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/cff54d05bcad Drop redundant calling start(). r=birtles