anchor.diff:  
   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";
   }
   
  
positive.code:  * This [Error] is thrown when an instance cannot implement one of the methods
   * in its signature.
   */
  class UnsupportedError implements Error {
    final String message;
    UnsupportedError(this.message);
    String toString() => "Unsupported operation: $message";
  
negative.code:  * This is a generic error used for a variety of different erroneous
   * actions. The message should be descriptive.
   */
  class StateError extends Error {
    final String message;
    StateError(this.message);
    String toString() => "Bad state: $message";
  