Warn when mutating props on a ReactElement

[sophiebits]

Test Plan: jest. Also used ballmer-peak in IE8 to verify that it still works.

[sebmarkbage]

I'm not sure if it's worth the perf and complexity hit on every property in the entire app. Especially since this pattern is often used to add properties rather than to mutate existing props.

Temporarily enabling Object.freeze (and strict mode) is a pretty good way to help debugging once you hit this issue.

We can probably remove the .props mutation check too.

[sophiebits]

You're suggesting keeping checkAndWarnForMutatedProps as I have it here but getting rid of all the defineProperty stuff?

[sebmarkbage]

Yes.

[sophiebits]

Okay, done.

[jimfb]

Ok, we are on a good track to have our codebase ready for this change. Anything that hasn't been fixed has tasks assigned (ie. remaining fixes in progress). Let's bring this PR back onto the front burner. Any CR comments remaining, or are we otherwise ready to merge?

[sophiebits]

Ready as far as I know. Just rebased.

[sophiebits]

@sebmarkbage review?

[sebmarkbage]

Ok.@zpao, heads up that we'll need to ignore this warning internally until we've cleaned up remaining callsites.