Type/Test HMRRuntime

[mischnic]

1. Should fix the error mentioned in Parcel 2 transformers "asset.addIncludedFile()" #5677 (reply in thread). This was introduced in the lazy-bundling PR, asset was a string in some cases here:

   parcel/packages/runtimes/hmr/src/loaders/hmr-runtime.js

   Line 261 in 8a4fa3e

   if (!asset.depsByBundle[bundle.HMR_BUNDLE_ID]) {

2. Flow-type the HMR runtime (which would have caught 1.)
3. Add some minimal tests for the HMR client runtime.

Related to: #2497

Closes #5962

[height]

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[parcel-benchmark]

Benchmark Results

Kitchen Sink 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

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Cached Bundles

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React HackerNews 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

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Cached Bundles

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AtlasKit Editor 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

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Cached Bundles

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Three.js 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

No bundles found, this is probably a failed build...

Cached Bundles

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