Replace stray "var" by "let"

Likely missed in bdf91b5941.

lib/compiler/passes/generate-js.js

@@ -428,7 +428,7 @@ function generateJS(ast, options) {
         } else {
           let values = Array(n);
 
-          for (var i = 0; i < n; i++) {
+          for (let i = 0; i < n; i++) {
             values[i] = s(this.sp - n + 1 + i);
           }