Observer checks for use of React.memo

[danielkcz]

Fixes #709

The React.memo returns an object which isn't expected and is passed down to observerClass which assumed a class component, but it got a plain object without a prototype present.

The solution works in a similar fashion as the one for React.forwardRef, but it simply removes additional React.memo by accessing a wrapped function component and applying observer to it. And it issues a warning.

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I've also installed @testing-library/react and jest-mock-console to start modernizing tests slightly. There was a small hiccup as we have TS 2.6 in here, but the latest peer dep @types/yargs is built on top of TS 3.5 (requiring unknown). I had to force the older version there.

[coveralls]

Coverage decreased (-0.2%) to 90.746% when pulling db95f90 on fix/check-for-memo into fc6eaa8 on master.

[urugator]

Looks good. I would prefer it to throw.
Can't comment on deps update.

[danielkcz]

I would prefer it to throw.

The throw would probably make sense if it wasn't possible to recover from it.

[urugator]

I don't want to argue about this, but imo either it's a valid use case then it shouldn't warn (at least on production) or it's not and then it should throw (so I am forced to fix it).

[mweststrate]

Is it allowed to use static members on memo components? Such as defaultProps, propTypes etc? If that is the case, we should throw instead of fixing, as we might accidentally loose those members.

[urugator]

It is facebook/react#14298 (comment)

[danielkcz]

Ok, you are right. Changed it to throw. Sadly, the test is worthless in that case, but I kept it there for a future reference when it's possible to properly test errors in React.

[danielkcz]

I think it's time to merge :)