chore: replace as many Object.assign with object spread as possible

[SimenB]

Summary

We use @babel/preset-env now, so this is safe.

Test plan

Green CI

[jeysal]

IIRC someone at v8 was working on it recently, but the last time I checked object spread was still slower than Object.assign in many cases. Did you notice any differences in performance?

[SimenB]

It's transpiled to babel's implementation anyways, so doesn't matter: https://github.com/babel/babel/blob/694e3fd8cf321988c0f1d97e59a3df77adabad5b/packages/babel-helpers/src/helpers.js#L387-L405

We could use {loose: true, useBultins: true} at which point it'd be transpiled to Object.assign.

That said, object-spread is faster than Object.assign in newer versions of V8, so whenever we drop node 6, we'll be ready 🙂

https://twitter.com/bmeurer/status/1015105293301280768?lang=en

[jeysal]

It's transpiled to babel's implementation anyways, so doesn't matter: babel/babel:packages/babel-helpers/src/helpers.js@694e3fd#L387-L405

Oh, for some reason I though object spread was introduced in Node 6, but I looked it up and it's 8+, so you're right, it'll be transpiled.
Anyway, it's definitely awesome for readability 👍

[SimenB]

Anyway, it's definitely awesome for readability

Yeah! Also easier for type systems (as you can see, I was able to remove quite a few $FlowFixMe comments 🙂)

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