Failure on cyclical type imports

[STRML]

Flow is able to handle cyclical imports, but once these type imports are reified into real imports, it can cause problems in real apps.

For example:

A.js

import type {Foo} from './B'
export default class Base {
  fn(a: Foo) {
    return a
  }
}

B.js

import A from './A'
export type Foo = Object;
export default class B extends A {}

In this case you'll get something like:

inherits.js:9 Uncaught TypeError: Super expression must either be null or a function, not undefined

because the superclass isn't defined yet due to the cycle.

I've added the stub of a test here.

Not sure how to handle this - for basic type exports, it could be mitigated by creating a new file, moving all typedefs into that file, then importing that file from the base and all importers, but for instanceof checks it's significantly trickier.