WebKit, which is the heart of Safari, is open source: http://webkit.org
I've been tracking many of these issues on the WebKit bug tracker: https://bug.webkit.org
For example: https://bug.webkit.org/show_bug.cgi?id=80559 (Bug 80559 - [ES6] Add support for ES6 Harmony).
The release notes linked to also links to the public commit that implemented each noted fix/update/new feature.