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);

}