Re-define ApplicationException.toString() in pub.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//352263002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37715 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/exceptions.dart

@@ -4,7 +4,6 @@
 
 library pub.exceptions;
 
-import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
@@ -23,6 +22,8 @@ class ApplicationException implements Exception {
   final String message;
 
   ApplicationException(this.message);
+
+  String toString() => message;
 }
 
 /// A class for exceptions that wrap other exceptions.