Running https://github.com/w3c/web-platform-tests/pull/4517 in Safari TP results in "item of sp is not a function". (I filed the formData() issue there separately.)
Thanks for letting us know. I'll take a look soon.
Created attachment 298590 [details] Patch
Created attachment 298592 [details] Patch
<rdar://problem/29970907>
Created attachment 298593 [details] Patch
Created attachment 298598 [details] Patch
Comment on attachment 298598 [details] Patch Clearing flags on attachment: 298598 Committed r210593: <http://trac.webkit.org/changeset/210593>
All reviewed patches have been landed. Closing bug.