anchor.diff:   *
    * Clients may not extend, implement or mix-in this class.
    */
  -abstract class StringInterpolation extends SingleStringLiteral {
  +abstract class StringInterpolation implements SingleStringLiteral {
     /**
      * Return the elements that will be composed to produce the resulting string.
      */
  
positive.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class PostfixExpression extends Expression
      implements MethodReferenceExpression {
    /**
     * Return the expression computing the operand for the operator.
     */
  
negative.code:  *
   * Clients may not extend, implement or mix-in this class.
   */
  abstract class IsExpression implements Expression {
    /**
     * Return the expression used to compute the value whose type is being tested.
     */
  