Update more diagnostic messages ES6->2015

[texastoland]

Fix #8996 CC @mhegazy.

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[texastoland]

Passed locally I must need to rebase…

[mhegazy]

thanks!

[mhegazy]

For future references, files under lib\ are generated by jake LKG, and not updated in PRs.

[texastoland]

Curious because those did look generated. So they're not intended to be versioned?

[mhegazy]

We refresh them regularly. but off master.

[texastoland]

Makes sense thanks!