Tail Calls: Non-simple parameter in strict mode 

Should we keep the use strict statement?

```javascript
repl: Non-simple parameter in strict mode (1:22)
> 1 | function factorial(n, acc = 1) {
    |                       ^
  2 |     'use strict';
  3 |     if (n <= 1) return acc;
  4 |     return factorial(n - 1, n * acc);
```