operator [] method

E operator [] (dynamic index)

Returns the value associated with property from the proxied JavaScript object.

The type of property must be either String or num.

Implementation 

E operator [](index) {
  // TODO(justinfagnani): fix the semantics for non-ints
  // dartbug.com/14605
  if (index is num && index == index.toInt()) {
    _checkIndex(index);
  }
  return super[index];
}