anchor.diff:   *
    * Clients may not extend, implement or mix-in this class.
    */
  -abstract class StringLiteral extends Literal {
  +abstract class StringLiteral implements Literal {
     /**
      * Return the value of the string literal, or `null` if the string is not a
      * constant string without any string interpolation.
  
positive.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class TypeParameterList extends AstNode {
    /**
     * Return the left angle bracket.
     */
  
negative.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class Statement implements AstNode {
    /**
     * If this is a labeled statement, return the unlabeled portion of the
     * statement, otherwise return the statement itself.
  