Netlify-cms: Add necessary polyfills to support transpiled features.

[tech4him1]

We transpile our JS code, but it doesn't necessarily support everything (see https://babeljs.io/docs/usage/caveats/). When babel-preset-env v7 is released, we can use useBuiltIns: "usage" to automatically add any polyfills needed after transpilation.

[tech4him1]

We can also remove the browser targets from the .babelrc, when we upgrade to v7, because they added support for using the browserslist in package.json.

[tech4him1]

I also think we should remove the stage-* presets and use the specific plugins we want directly.

[tech4him1]

Babel 7 also affects the transform-runtime plugin: useBuiltIns is added, which means we can stop using polyfilled Map, Symbol, etc, by setting it to true.

EDIT: Actually the new useBuiltIns: "usage" setting in the ENV preset is the same as that part of transform-runtime, so we would want to have both settings on, which turns the transform-runtime one off, and leaves the ENV one on.

EDIT: Actually, they're not exactly the same. The ENV one pulls in only the polyfills that are needed for the supported browsers, but it adds them to the global scope. Transform-runtime adds them for everything, even when supported, but only adds them to the module scope. See https://github.com/babel/babel/issues/7267 for details.

[tech4him1]

If the discussion in https://github.com/babel/babel/issues/6628 takes off, that would be nice.

[tech4him1]

Just did some testing with Babel v7.0.0-beta.38. transform-runtime has the benefit of not polluting the global scope, but IMO it's not the most maintainable/minimal option, since it polyfills everything, even if the browser supports it.

I propose that we do:

["@babel/preset-env", {
  "modules": false,
  "useBuiltIns": "usage" // Add necessary polyfills to the global scope.
}]

Only use the runtime transform for helpers. We probably don't really need this anyway -- we're not transpiling much anyway with only the last 2 versions of browsers -- so it probably won't really change size.

["@babel/plugin-transform-runtime", {
  "helpers": true,
  "useBuiltIns": true,
  "useESModules": true,
  "polyfill": false,
  "regenerator": false
}]

[erquhart]

I'm with that.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[erezrokah]

Closing this as outdated. Please re-open if necessary.