All of the browsers we currently support, as specified by the
`browserslist` key in package.json, fully support ES 2015 and ES 2016.

The only exception I'm aware of is that Edge Legacy v17 and v18 do not
support using string regex search/match methods with custom (non-regex)
types via `Symbol.{match, search, replace}`. That's not something we've
ever used AFAIK.