Observable.from should throw primitive iterables (strings)

[benlesh]

Along with how Iterator helpers behave, if a user passes a string, even though it's an iterable, it's likely an error. We should throw a TypeError in that case.

Observable.from('some string'); // TypeError

[bakkot]

I think you misread my comment (which was not all that clear, sorry). Iterator.from does not throw when given a string. This makes it different from flatMap, which does throw when the mapper returns a string, even though strings are iterable.

See discussion in tc39/proposal-iterator-helpers#244.