Use const instead of let

[mbelsky]

Hey,
With eslint's prefer-const rule I've found a few variables that declared as let but them never rewritten. Does this project have any guidelines to take let or const?

[msftclas]

All CLA requirements met.

[itsmunim]

@RMacfarlane won't it be better to do a rebase with master instead of merging master into it and having a merge commit for that? Not a great history to look at!

git pull --rebase upstream master

[RMacfarlane]

@dibosh I typically use squash when merging something back into master, so it should ultimately be fine. That merge commit was from hitting the "Update branch" button that GitHub now exposes.

[RMacfarlane]

@mbelsky This project doesn't have guidelines for let vs const, but I think this change is good so I've merged it in. Thanks!