function CallbackProperty(callback, isConstant) {
  this._callback = undefined;
  this._isConstant = undefined;
  this._definitionChanged = new Event();
  this.setCallback(callback, isConstant);
}
A {@link Property} whose value is lazily evaluated by a callback function.

@alias CallbackProperty
@constructor

@param {CallbackProperty.Callback} callback The function to be called when the property is evaluated.
@param {boolean} isConstant <code>true</code> when the callback function returns the same value every time, <code>false</code> if the value will change.