Should Observable.from use a brand check to confirm argument is Observable
Observable.from currently only checks that the constructor property of the input is equal to the the this object of
Observable.from. This seems easy to spoof. It's straightforward to create an object with a
constructor property equal to the Observable constructor that does not contain a
subscribe method or otherwise behave like an Observable. Should we use a brand check to confirm that the input to Observable.from is indeed a species of Observable?