code: function getDefaultValue(component, propertyName, source) {
    if (!propertyName) {
      return component.schema.default;
    }
    return component.schema[propertyName].default;
  }
docs: Gets the value for a component or component's property coming from schema
  defaults.
  
  @param {Component} component      Component to be found.
  @param {string}    [propertyName] If provided, component's property to be
  found.
  @param {Element}   source         Element owning the component.
  @return                           The component value coming from the schema
  default.