Convert NoMoreElementsException and EmptyQueueException to StateError.

lib/core/errors.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class Error {
   const Error();
 }
 
 /**
  * Error thrown by the runtime system when an assert statement fails.
@@ skipping 138 matching lines @@
 
 /**
  * The operation was not allowed by the object.
  *
  * This [Error] is thrown when a class cannot implement
  * one of the methods in its signature.
  */
 class UnsupportedError implements Error {
   final String message;
   UnsupportedError(this.message);
-  String toString() => message;
+  String toString() => "Unsupported operation: message";
 }
 
 /**
  * The operation was not allowed by the current state of the object.
  *
  * This is a generic error used for a variety of different erroneous
  * actions. The message should be descriptive.
  */
 class StateError implements Error {
   final String message;
   StateError(this.message);
-  String toString() => message;
+  String toString() => "Bad state: message";
 }
 
 
 class OutOfMemoryError implements Error {
   const OutOfMemoryError();
   String toString() => "Out of Memory";
 }
 
 class StackOverflowError implements Error {
   const StackOverflowError();
   String toString() => "Stack Overflow";
 }