Subject.create vs AnonymousSubject.create

[benlesh]

Related #2002, #2003, #1996, #1975

Currently, Subject.create is built to mimic it's behavior in v4 and under. That is that Subject.create actually returns an AnonymousSubject, and not a Subject.

Problem

• This flies in the face of other uses of create in this library, which is as an alternative to new... for example: new Observable(fn) vs Observable.create(fn). This creates confusion for users (and experts, apparently)
• We can't move AnonymousSubject into it's own module, because there would be a circular reference with Subject, which has this create method that only creates an AnonymousSubject.

Proposal:

• Subject.create becomes alternative version of new Subject
• AnonymousSubject.create does what Subject.create does now... which is just an alternative version to new AnonymousSubject.
• move AnonymousSubject to it's own module.

[reduckted]

This is really confusing behaviour. I figured, since new Observable() and Observable.create() are basically the same, that new Subject() and Subject.create() would also be the same. How wrong I was!

[zoechi]

#2002 was merged but that doesn't make AnonymousSubject public. For this it would need to be listed here as well https://github.com/ReactiveX/rxjs/blob/master/src/index.ts#L10.
Also new AnonymouseSubject(...) supports a destination and source parameter, while new Subject() or Subject.create() do not. Some comment on how to translate would be great when one is deprecated.