Test emitting let instead of var.

Fixes #856

Author: cristianoc

jscomp/build_tests/custom_namespace/src/demo.bs.js

@@ -1,7 +1,7 @@
 'use strict';
 
 
-var v = 42;
+let v = 42;
 
 exports.v = v;
 /* No side effect */

jscomp/build_tests/devonly/src/demo.bs.js

@@ -29,7 +29,7 @@ let function_ = "function"
 
 let function_async ~async = if async then "async function" else "function"
 
-let var = "var" (* should be able to switch to [let] easily*)
+let var = "let" (* should be able to switch to [let] easily*)
 
 let return = "return"