Use var instead of const to handle all browsers with helpers

[disnet]

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()');