Flow upgrade

[SimenB]

Summary
Tried to upgrade, and using flow-upgrade, but there's still tons of errors in pretty-format tests. I'm guessing I shouldn't just litter the tests with FlowFixMes?

/cc @pedrottimark

Test plan
yarn flow (currently failing)

[pedrottimark]

@SimenB Although I am happy to make these changes, it might be more efficient for you:

• For React.createElement(undefined) let’s mock the object to eliminate console warning. I think you will need to rebase because of Return UNDEFINED if type neither string nor function in ReactElement plugin #4360 though. While you are at it, can you please replace Unknown element with undefined element type in the test message?

  const elementSymbol = Symbol.for('react.element'); // ADD preceding the following line:
  const testSymbol = Symbol.for('react.test.json')
  
  test('supports undefined element type', () => {
    expect(formatElement({$$typeof: elementSymbol, props: {}})).toEqual(
      '<UNDEFINED />',
    );
  });

• null props is reported and labelled as bug react in Flow issues: Error when using React.createElement directly facebook/flow#4658 but no errors for {} empty object, so an alternative work-around to $FlowFuxMe comments (couldn’t resist :) is add const NULL = {}; near the beginning of the file and replace all those null with NULL

[SimenB]

Thanks for the help @pedrottimark!

I chose to use $FlowFixMe for the null bug, so that it can be easily removed whenever flow fixes it in the future (I love how flow errors on ignores that do nothing)

[codecov-io]

Codecov Report

Merging #4355 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4355   +/-   ##
=======================================
  Coverage   55.85%   55.85%           
=======================================
  Files         189      189           
  Lines        6383     6383           
  Branches        6        6           
=======================================
  Hits         3565     3565           
  Misses       2815     2815           
  Partials        3        3

Impacted Files	Coverage Δ
...ackages/pretty-format/src/plugins/react_element.js	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update db4465e...ddefa03. Read the comment docs.

[pedrottimark]

You’re welcome. Thank you for explaining. Y’all have a lot to teach me about fluent JS and Flow.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.
Please note this issue tracker is not a help forum. We recommend using StackOverflow or our discord channel for questions.