Roll Observatory packages and add a roll script

packages/path/lib/src/path_exception.dart

 // Copyright (c) 2013, 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.
 
 library path.path_exception;
 
 /// An exception class that's thrown when a path operation is unable to be
 /// computed accurately.
 class PathException implements Exception {
   String message;
 
   PathException(this.message);
 
   String toString() => "PathException: $message";
 }