src/list.js : TODO: remove this when transpiler shenanigans are over

from `src/list.js` : 

``` es6

static of(subTypes) {
        //TODO: remove this when transpiler shenanigans are over
        if (arguments.length > 1) {
            subTypes = Array.prototype.slice.call(arguments);
        }
        return this.withDefault(undefined, undefined, { subTypes });
    };
```

@tobich 
