Add yarn build --unsafe-partial

[gaearon]

We normally delete the build folder when rebuilding for consistency. This can be annoying if I make a tiny scoped edit to a particular package. This is an escape hatch so I can iterate faster when I know what I'm doing.

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[bvaughn]

Oooh this would be very nice!

I think we should still rimraf the specific sub-folders though, so they're not left in a funky state.

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[gaearon]

You think so? I actually like the entry point granularity.

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[bvaughn]

The case I'd like to avoid is one of us testing an outdated/unrealistic local node_module (with stuff left over in the package from a previous build).

So e.g.

yarn build -- react,react-dom --type=NODE      

I think it would be safer to start with

 rm -rf build/node_modules/react
 rm -rf build/node_modules/react-dom

But I agree with the idea of not rm -rf build because that makes it much slower to incrementally test things locally.

Maybe the thing I'm really concerned about is the (unlikely) case that any of us would actually publish to NPM after a partial build- which could be problematic even if we were clearing the package sub-folders, so I dunno...

[gaearon]

Maybe I'll rename to partial-dangerous? I really don't intend this to be used widely. Only for cases where you're debugging why some prod test is failing or similar.

[gaearon]

Changed to unsafe-partial.

[bvaughn]

Our codebase is a minefield, so many "unsafe" thigns 😆

¯\_(ツ)_/¯ Seems fine. Will save some time.