Observable subscriptions from previous validation runs should be canceled
before a new subscription is created for the next validation run.
Currently the subscription that sets the errors is canceled properly,
but the source observable created by the validator is not. While this
does not affect validation status or error setting, the source
observables will incorrectly continue through the pipeline until they
complete. This change ensures that the whole stream is canceled.
AsyncValidatorFn previously had an "any" return type, but now it more
explicitly requires a Promise or Observable return type. We don't
anticipate this causing problems given that any other return type
would have caused a runtime error already.
This API was introduced only in a beta release, and is being removed because we found it to be incorrect prior to launch. For more information about why this is being removed, see https://github.com/angular/angular/pull/15050.
Jason Aden
Kara