DevTools: Inline references to fiber flags

[acdlite]

We shouldn't be referencing internal fields like fiber's flag directly of DevTools. It's an implementation detail. However, over the years a few of these have snuck in. Because of how DevTools is currently shipped, where it's expected to be backwards compatible with older versions of React, this prevents us from refactoring those fields inside the reconciler.

The plan we have to address this is to fix how DevTools is shipped: DevTools will be released in lockstep with each version of React.

Until then, though, I need a temporary solution because it's blocking a feature I'm working on. So in meantime, I'm going to have to fork the DevTool's code based on the React version, like we already do with the fiber TypeOfWork enum.

As a first step, I've inlined all the references to fiber flags into the specific call sites where they are used. Eventually we'll import these functions from the reconciler so they stay in sync, rather than maintaining duplicate copies of the logic.

[mondaychen]

Would it be better if all of these flags are defined in the same place at the top of this file?

1. they are easier to maintain in case we had to keep them longer than expected
2. we won't miss any of them when we move those functions to reconciler

[acdlite]

I inlined them intentionally because even the concept of a flag shouldn't have leaked into DevTools. Ideally we wouldn't even leak the concept of a Fiber. DevTools should be interacting with higher level concepts like "is this component in an error state?" and "did this component perform any work during the last render?", rather than leaking internal types.

It's bad that these leaked in the first place but I'd rather confine them to the actual place where they are referenced instead of encouraging these references to spread even more.

[acdlite]

In other words, rather than fork the flag itself (e.g. Placement), we should be forking the implementation of the function they are used in (e.g. getNearestMountedFiber).

Then once the DevTools packaging is fixed, the final step is to import getNearestMountedFiber directly from React, rather than maintain a duplicate copy.

[mondaychen]

I see, that makes sense!