Consider relaxing tslib dependency or upgrade it.

[OnurGumus]

Currently rxjs relies on ~2.1.0 for tslib. I find this restriction peculiar and also limiting as newer version of tslib is 2.3.1

I wonder if we could relax the version constraint to maybe ^2.1.0 or upgrade it to ~2.3.0?

[kwonoj]

Closing the loop, this is in now. #6692