CodeSandbox CI builds don't include JS

[raunofreiberg]

This is not related to the components themselves, but rather the Codesandbox PR deployments.

Take a look at https://codesandbox.io/s/hardcore-hertz-fu7d8. Notice that there's an error when importing components from @reach/tabs (this applies to all packages).

On further inspection, downloading the package (https://pkg.csb.dev/reach/reach-ui/commit/61bcce10/@reach/tabs") via Codesandbox and opening it reveals that it does not include any JS:

➜  Downloads ls package
README.md    package.json styles.css

I am not sure what the problem here is but it's something we should definitely fix.

[raunofreiberg]

To be continued...

It seems like usage of Array.flatMap is causing the error here:

reach-ui/scripts/utils.ts

Line 70 in fd8102b

.flatMap(file => glob(file));

Inspecting the CI build of Codesandbox throws an error about it for every package:

@reach/utils: (node:224) UnhandledPromiseRejectionWarning: TypeError: _b.apply(...).flatMap is not a function
    at /tmp/01efb6d3/scripts/utils.ts:70:6

[raunofreiberg]

Apparently, for now, there is no way to change the Node version (10.18.1) inside the CI container:
https://twitter.com/codesandbox/status/1262349909216370688

Let's just check for flatMap instead and fall back to an alternative implementation.