`let x=1` and `const x=1` don't work.

package m

import (
	"fmt"
	"testing"

	"github.com/robertkrimen/otto"
)

func TestEvalJs(t *testing.T) {
	vm := otto.New()
	vm.Set("body", `{"page":1}`)
	v, err := vm.Run(`
        let abc = 2 + 2;// Not work here
        console.log("The value of abc is " + abc); // 4
    `)
	if err != nil {
		fmt.Printf("%+v, val:%v", err, v)
	}
}