x/tools/cmd/present: Fix present not working on Safari
x/tools/cmd/present: Fix present not working on Safari

As described on https://github.com/golang/go/issues/16216 present is not
currently working on Safari because of the use of ES6 JavaScript let
statements.
The solution is to use var statements instead of let.

Fixes golang/go#16216

Change-Id: I44c84f920aa41dfc8de5997ab50a440de6d57093
Reviewed-on: https://go-review.googlesource.com/24721
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,golang/tools