repl autocompletion is off for `const` and `let` variables

- **Version**: v6.4.0
- **Platform**: Linux _ 4.4.0-34-generic #53-Ubuntu SMP Wed Jul 27 16:06:39 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
- **Subsystem**: repl

if you define variable `foo` like this `foo = 123` then autocompletion will show `foo` in the list.

However, if `foo` are defined like this `let qwe = 123` or `const asd = 123` then  autocompletion will be off for this vars.
