anchor.diff:   */
   class NullThrownError implements Error {
     const NullThrownError();
  +  /** NullThrown errors don't capture stack traces. */
  +  StackTrace get stackTrace => null;
     String toString() => "Throw of null.";
   }
   
   /**
    * Error thrown when a function is passed an unacceptable argument.
    */
  -class ArgumentError implements Error {
  +class ArgumentError extends Error {
     final message;
   
     /** The [message] describes the erroneous argument. */
  
positive.code:  * an [UnsupportedError] instead. This error is only intended for
   * use during development.
   */
  class UnimplementedError implements UnsupportedError {
    final String message;
    UnimplementedError([String this.message]);
    String toString() => (this.message != null
                          ? "UnimplementedError: $message"
                          : "UnimplementedError");
  
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";
  }
  
  