Only stop if the transition on this element ended

This way it doesn't end when some random element elsewhere on the page has an ending transition.

It must be noted that this is not a complete fix. If you have multiple transitions on the element, it still ends when the first ends.

I ran into this not working (the transition ends almost immediately, because some unrelated transition ends). This fixes it for me, but I was wondering whether this cornercase was considered, and whether the other cornercase (multiple transitions) was considered.

I think it's very hard to get this correct, and think it might be better to make the timeout explicit, or at least allow to set it explicit, and fall back to this behaviour. But I'm really interested in feedback on this one.
Guard against contentEditable with children props
Added event parameter
Make sure it works when we have no css transitions
add picture element and related attributes
add picture to xjs.knownTags
Add support for <dialog> tag.

fixes #2017
use attribute for media, srcSet and sizes
Improve debugging for methods using ReactPerf
Combine displayName using underscore
Add newline before JSX spread when appropriate

Previously, we were losing the newline before a JSX spread attribute which would cause the lines to not match up; this fixes the problem.
Add support for <dialog> tag's open attribute.
Fix DOMProperty bitmask checking
Fix broken link in animation docs

Missed in #2068
Merge pull request #2033 from pekim/master

Add support for <dialog> tag.
JSX spec blog post
Merge pull request #2106 from subtleGradient/subtleGradient/improve-ReactPerf-debugging

Improve debugging for methods using ReactPerf
Comment typo
added the missing **

Missed it the last time. Apologies.
Update Like button example verbiage

Changes text "You unlike this" to "You liked this". Makes a bit more grammatical sense.

Closes #2146.