remove non-observable side effects code

The output below is correct, `a` seems to be a global exception id, however, such side effect is not observable, is it possible to remove such code(ideally, I should get an empty output)

``` js
// Input 0./caml_exceptions.js
'use strict';
// Input 1./caml_array.js
// Input 2./caml_primitive.js
// Input 3./caml_utils.js
// Input 4./caml_io.js
// Input 5./camlinternalFormatBasics.js
// Input 6./pervasives.js
var a = 0;
a++;
// Input 7./array.js
a++;
```