Chrome has had them for a while. Supposedly they just hid them behind a flag because they weren't satisfied with some perceived deficiencies in the spec. (Check out https://v8project.blogspot.com/2016/04/es6-es7-and-beyond.ht...) WebKit did have it somewhere between mid-October to early December of 2015. (http://web.archive.org/web/20151207203829/http://kangax.gith...) Chrome didn't get it until late April of the next year (http://web.archive.org/web/20160526102037/http://kangax.gith...), though there's no snapshot until late May. People worked on the spec in a different order.

Chrome also had a failing array prototype under some non-standard syntax for a while. Despite claiming 100%, Kangax didn't show them passing. Test results have fluctuated as the tests themselves were refined. I've seen some passing results later turn into fails. That's why none of the browsers are at 100% for ES6 currently.

I'm still grudging at Apple for the year's delay between Safari 9 and 10, which left all the clueless Apple customers on ~53% ES6. Yes, WebKit had regular updates, but they didn't do a gradual release and few people thought to install WebKit instead of Safari (or even the TP). Other browsers had much better beta channels than Apple. And didn't pointlessly delay on things like WebRTC. Or block other browser engines like iOS to protect their app store.

Chrome did a good job of releasing updates gradually, but there are still some ES6 performance issues to be ironed out.