anchor.diff:   *
    * Clients may not extend, implement or mix-in this class.
    */
  -abstract class ConstructorInitializer extends AstNode {}
  +abstract class ConstructorInitializer implements AstNode {}
   
   /**
    * The name of a constructor.
  
positive.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class InterpolationExpression extends InterpolationElement {
    /**
     * Return the expression to be evaluated for the value to be converted into a
     * string.
  
negative.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class ExtendsClause implements AstNode {
    /**
     * Return the token representing the 'extends' keyword.
     */
  