anchor.diff:   *
    * Clients may not extend, implement or mix-in this class.
    */
  -abstract class ImportDirective extends NamespaceDirective {
  +abstract class ImportDirective implements NamespaceDirective {
     static Comparator<ImportDirective> COMPARATOR =
         (ImportDirective import1, ImportDirective import2) {
       //
  
positive.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class AdjacentStrings extends StringLiteral {
    /**
     * Return the strings that are implicitly concatenated.
     */
  
negative.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class ClassDeclaration implements ClassOrMixinDeclaration {
    /**
     * Return the 'abstract' keyword, or `null` if the keyword was absent.
     */
  