[flow] type KeyEscapeUtils (#7079)
[flow] type KeyEscapeUtils (#7079)

Summary:

The only call site ensures that the value is not null: https://github.com/facebook/react/blob/dc6fc8cc0726458a14f0544a30514af208d0098b/src/shared/utils/traverseAllChildren.js#L44

key is stringified inside of createElement, so we are guaranteed to receive a string right now: https://github.com/facebook/react/blob/dc6fc8cc0726458a14f0544a30514af208d0098b/src/isomorphic/classic/element/ReactElement.js#L142

Test Plan:
npm run flow

Reviewers: 663c8c2f2861ffa32b57d2756c3a277500005cdc@spicyj
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