Add useBuiltIns babel-preset-env option (fix #843)

[samsch]

Enables babel-preset-env useBuiltIns option for source packages (and not node_modules packages). Fix for #843

No tests were harmed in the making of this pull request. (Prettier got excited and added a newline though.)

I added babel-polyfill as an explicit devDependency for a test.

As per any test dependent on external sources (browserlist), this isn't perfectly future-proof.

[codecov-io]

Codecov Report

Merging #878 into master will decrease coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #878      +/-   ##
=========================================
- Coverage   91.98%   91.7%   -0.28%     
=========================================
  Files          68      68              
  Lines        3404    3494      +90     
=========================================
+ Hits         3131    3204      +73     
- Misses        273     290      +17

Impacted Files	Coverage Δ
src/transforms/babel.js	95.52% <100%> (+0.16%)	⬆️
src/assets/CSSAsset.js	83.47% <0%> (-10.13%)	⬇️
src/utils/getTargetEngines.js	96.59% <0%> (-2.15%)	⬇️
src/visitors/fs.js	89.51% <0%> (-2.09%)	⬇️
src/assets/HTMLAsset.js	97.8% <0%> (-1.3%)	⬇️
src/visitors/dependencies.js	93.5% <0%> (+0.35%)	⬆️
src/utils/config.js	91.3% <0%> (+0.82%)	⬆️
src/utils/localRequire.js	96.66% <0%> (+0.83%)	⬆️
src/assets/JSAsset.js	94.53% <0%> (+1.93%)	⬆️
... and 3 more

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[2hu12]

According to the babel preset-env doc, the default value of useBuiltIns is false, so what about make the second parameter of getEnvPlugins with a default false, like

async function getEnvPlugins(targets, useBuiltIns = false) { ... }

[samsch]

There does seem to be a little precedent to use a default argument, and it does seem to make usage a bit clearer. This does not actually change any functionality, since the argument is used for a truthy check, and undefined is falsy.

[FDiskas]

This will fix errors like src/index.js: Unknown version 62 of android ?

[samsch]

@FDiskas Probably not, no. This shouldn't fix any errors, it decreases the bundle size by replaying an import of babel-polyfill with separate imports of just the necessary polyfills.

[devongovett]

Thanks for doing this. When we upgrade to Babel 7, we should also switch to the new usage option for useBuiltins which makes it so you don't even need to import babel-polyfill at all.