Fix methods of LazyPromise

[jlfwong]

I was running into problems using code splitting, and eventually discovered it was because of a bug in LazyPromise.

If you have code like this:

const foo = import('./foo')
foo.then(() => console.log('1'))
foo.then(() => console.log('2'))

Then I would expect the the file would log both '1' and '2'. Before this fix, it logs only '1', because if the promise executor has already been invoked, then LazyPromise.prototype.then is a no-op, just returning the existing promise.

The fix is to always invoke the appropriate callback, even if the promise has already been executed.

I ran into this issue when doing code splitting because I wanted to eagerly, but asynchronously load a dependency, and then use it later. I suspect this problem will also occur in any situation where a dynamic import() is used in multiple places in a single file.

[jlfwong]

Thanks for the review, @fathyb! Is there anything else I need to do to get this merged?