Access object properties using brackets

Is there a way to access object properties using brackets? Currently, it seems there's no way to access an object property that has spaces on its name. e.g.

```
let expr = Parser.parse('a["property with spaces"]');
console.log(expr.evaluate({ a: { 'property with spaces': 5 } }));
```

throws

```
Error: parse error [1:2]: Unknown character "["
```