When a single resource is preloaded twice in ngtsc, the second request
would be recognized as in-flight in which case `undefined` would
be returned, which signals to the compilation that is can resume
synchronously. The compilation would then proceed immediately and call
`load`, only to find out that the request is still in-flight which is
not allowed.
This commit caches the Promise of the in-flight fetch requests, such
that subsequent preload requests can return the corresponding Promise
instance.
PR Close#27357
b5ed403bc4