Modernize distributed bundle

[anandthakker]

IE 11 is the only browser we target that doesn't support the ES2015 that we currently compile to ES5:

Via babel-preset-env. According to MDN, Edge does support basic destructuring, just not 'rest' for arrays/objects.

I propose that we:

• Modernize our main bundle (dist/mapbox-gl.js, https://api.tiles.mapbox.com/mapbox-gl-js/VERSION/mapbox-gl.js) to un-transpiled ES2015 code.
• Continue to support IE 11 by distributing a separate compatibility bundle as dist/mapbox-gl.ie11.js / https://api.tiles.mapbox.com/mapbox-gl-js/VERSION/mapbox-gl.ie11.js.

There are benefits to a modernized bundle for users who don't need IE 11 support:

• It'll be a bit smaller -- not tons, but with a 500K+ bundle, every bit counts.
• If it's not already faster, I think it will be soon: it seems pretty likely that JS engines will start including optimizations that are specific to ES2015 constructs that wouldn't apply to their ES5-compiled counterparts.

(We could also do this the other way: keep dist/mapbox-gl.js as-is, but start destributing dist/mapbox-gl.es2015.js or something)

[andrewharvey]

While I like the idea, I think it makes more sense to be opt in (es2015 build) for the time being rather than opt out (ie11 build). I still see a fair amount of users on IE11 or on outdated Chrome/Firefox builds.

[anandthakker]

it seems pretty likely that JS engines will start including optimizations that are specific to ES2015 constructs that wouldn't apply to their ES5-compiled counterparts.

One example of such an optimization: looks like object spread syntax will be faster than a transpiled equivalent (or Object.assign) in v8: https://twitter.com/bmeurer/status/1015105293301280768