An Expando allows adding new properties to objects.

class Expando<T> {

  /**
   * The name of the this [Expando] as passed to the constructor. If
   * no name was passed to the constructor, the name is [null].
   */
  final String name;

  /**
   * Creates a new [Expando]. The optional name is only used for
   * debugging purposes and creating two different [Expando]s with the
   * same name yields two [Expando]s that work on different properties
   * of the objects they are used on.
   */
  external Expando([String name]);

  /**
   * Expando toString method override.
   */
  String toString() => "Expando:$name";

  /**
   * Gets the value of this [Expando]'s property on the given
   * object. If the object hasn't been expanded, the method returns
   * [null].
   */
  external T operator [](Object object);

  /**
   * Sets the value of this [Expando]'s property on the given
   * object. Properties can effectively be removed again by setting
   * their value to null.
   */
  external void operator []=(Object object, T value);

}