anchor.diff:   * This is a generic error used for a variety of different erroneous
    * actions. The message should be descriptive.
    */
  -class StateError implements Error {
  +class StateError extends Error {
     final String message;
     StateError(this.message);
     String toString() => "Bad state: $message";
  
positive.code:  * ([Iterable] or similar collection of values) should declare which operations
   * are allowed during an iteration.
   */
  class ConcurrentModificationError implements Error {
    /** The object that was modified in an incompatible way. */
    final Object modifiedObject;
  
    const ConcurrentModificationError([this.modifiedObject]);
  
    String toString() {
      if (modifiedObject == null) {
  
negative.code: 
  class OutOfMemoryError implements Error {
    const OutOfMemoryError();
    StackTrace get stackTrace => null;
    String toString() => "Out of Memory";
  }
  
  
  class StackOverflowError implements Error {
    const StackOverflowError();
    StackTrace get stackTrace => null;
    String toString() => "Stack Overflow";
  }
  
  