This pull request completely replaces our old page loader with a brand new route loader.
Our existing comprehensive test suite means I did not need to add a bunch of tests. I did add them where behavior was added or fixed.
Summary of the changes:
- Eagerly evaluates prefetched pages in browser idle time (speeds up transitions)
- Router is **no longer frozen** indefinitely if the Build Manifest never arrives
- Router is **no longer frozen** indefinitely if a page fails to bootstrap
- New `withFuture` utility instead of ad-hoc deduping per resource
- Prefetching is now delayed until browser idle time to not impact TTI
- Browsers without `prefetch` now fall back to eager evaluation instead of using `preload`
- We're now ready to serve non-static assets **with `no-store` without breaking prefetching**
- **Application can now hydrate without fetching CSS assets—this is a huge performance win that was previously blocking hydration**
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The minor size increase here is unfortunate, but we have to incur it for correctness.
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Fixes#18389Fixes#18642
