dynamic operator [](property)

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

The type of property must be either String or num.

Source 

operator [](property) {
  try {
    return _operator_getterLegacy(property);
  } catch (e) {
    // Re-throw any errors (returned as a string) as a DomException.
    throw new html.DomException.jsInterop(e);
  }
}