Advance to stage 4

[mathiasbynens]

Criteria taken from the TC39 process document minus those from previous stages:

• Test262 acceptance tests have been written for mainline usage scenarios, and merged

TODO

• Two compatible implementations which pass the acceptance tests

https://github.com/mathiasbynens/proposal-regexp-unicode-sequence-properties#implementations

Bug tickets to track:

• Chrome/V8: https://bugs.chromium.org/p/v8/issues/detail?id=7467
• Edge/Chakra: TODO
• Firefox/SpiderMonkey: https://bugzilla.mozilla.org/show_bug.cgi?id=1519168
• Safari/JavaScriptCore: TODO

• Significant in-the-field experience with shipping implementations, such as that provided by two independent VMs

https://github.com/mathiasbynens/proposal-regexp-unicode-sequence-properties#implementations

• A pull request has been sent to https://github.com/tc39/ecma262 with the integrated spec text

TODO

• The ECMAScript editor has signed off on the pull request

TODO