In my research, I found the data used by @babel/preset-env to be more
conservative than necessary, so I have not followed their minimum version
constraints exactly.
For example, every feature of the ECMAScript `Map` API is clearly
supported in Firefox 45+, according to MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map#Browser_compatibility
However, @babel/preset-env requires `core-js/modules/es6.map` in any
version of Firefox earlier than 53:
e270fbe7f0/packages/babel-preset-env/data/built-ins.json (L117)
Since I can't find any evidence that @babel/preset-env knows better than
other sources (I think they might just be using a compatibility table that
doesn't go back far enough), I have tentatively trusted MDN in picking
these versions.
If any bugs are ever reported due to this choice of versions and
polyfills, we have two options to fix them:
* Tighten the minimum version constraints so that the affected browsers
are considered legacy instead of modern.
* Include the missing `core-js` polyfills for all modern browsers.
