dynamic operator []=(property, value)

Sets the value associated with property on the proxied JavaScript object.

The type of property must be either String or num.

Source 

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