Flow: Support $FlowFixMe in the REPL

[ctrlplusb]

Please. :)

I am trying to create some cheatsheets and this would be helpful to have so that I can show cases where we should expect a Flow error to occur.

[gabro]

I would love this too! 👍

// $ExpectError

as in the docs would probably be even better

[gabro]

I tried to take a stab at this, but I wasn't able to inject any custom configuration in tryFlow.js.

What I tried is calling registerFile passing a .flowconfig content, but it doesn't work. I could definitely use a pointer here /cc @mroch

[gcanti]

This would be very helpful in order to share tests