[react-loadable] Make type application of ReactLoadable.Map more precisely

[kogai]

Why

When a programmer uses Loadable.Map, he faces a situation where a loaded module hasn't any type-restriction.
(Actually, it may evaluate as empty)

Then, the programmer applies its own type arguments into Loadable.Map like below.

Loadable.Map<Props, { a: () => Promise<string>, b: () => Promise<number> }>({...})

But this application causes type error with a message like Cannot instantiate MapOptionsbecause number [1] is incompatible with string [2] in propertybof type argumentLoadable.Map..

This causes from a restriction of type argument named TModules.
It defined as MapOptions<TProps, TModules: { [key: string]: * }>,
When type arguments applied to TModules, It seems Flow may infer resctriction of TModules like {[string]: () => Promise<string> & () => Promise<number>}, of course actual value like this can't exist anyay.

Therefore, it would be better to relax restriction of TModules to a type of properties as covariant.
It makes Flow can infer TModules more pragmatically.

FYI: A situation described above can reproduce more simply as below.

type T<S: {[string]: * }> = S;
declare var t: T<{a: string, b: number}>;

(t.a); // Cause error

Try code.

[kogai]

Whoops, something occurred at Travis 🤔
Several PRs seems to face this problem, too 👀
(In my local environment, all specs are green)

#2933 (comment)
#2931 (comment)

It seems tracking at #2916

[AndrewSouthpaw]

LGTM, thanks for the PR. Verified tests are green locally.

Nice work on the covariant inference, I didn't know that trick!