add ui:list test for W-1679646, also fix 'helper not found' issue triggered by abstractListHelper.js
@rev rjayaramappa@
@bug W-1679646@
Merge branch '186/patch' of git.soma.salesforce.com:aura/aura into 186/patch
Merging origin/lin/patch-pending-W-1784656 into 186/patch