If I leave it out I get the following:

```
./traceur --out test/unit/semantics/FreeVariableChecker.generated.js -- test/unit/es6/semantics/FreeVariableChecker.js

undefined:2127
var $__27 = System.get(arguments[0]);
                       ^
ReferenceError: arguments is not defined
    at eval (eval at <anonymous> (/Users/guybedford/Projects/traceur-compiler/src/node/traceur.js:24:17), <anonymous>:2127:24)
    at eval (native)
    at Object.<anonymous> (/Users/guybedford/Projects/traceur-compiler/src/node/traceur.js:24:17)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/Users/guybedford/Projects/traceur-compiler/src/node/command.js:29:15)
make: *** [test/unit/semantics/FreeVariableChecker.generated.js] Error 8
```