Use var instead of const to handle all browsers with helpers

Even modern browsers like Safari 10 apparently have problems with const in eval so just don't use ES2015 features.

The minimal repro for those interested. Throws a reference error in safari 10 (and tech preview). Same in recent-ish chrome too I think.

eval(`
const a = 1; 
function f() {
 console.log(a); 
}; 
f()');