Fix OAuth2 tests, and disable them on non-dart:io platforms.

pkg/oauth2/lib/src/authorization_exception.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.
 
 library authorization_exception;
 
 import 'dart:io';
+import 'dart:uri';
 
 /// An exception raised when OAuth2 authorization fails.
 class AuthorizationException implements Exception {
   /// The name of the error. Possible names are enumerated in [the spec][].
   ///
   /// [the spec]: http://tools.ietf.org/html/draft-ietf-oauth-v2-31#section-5.2
   final String error;
 
   /// The description of the error, provided by the server. Defaults to null.
   final String description;
 
   /// A URI for a page that describes the error in more detail, provided by the
   /// server. Defaults to null.
   final Uri uri;
 
   /// Creates an AuthorizationException.
   AuthorizationException(this.error, this.description, this.uri);
 
   /// Provides a string description of the AuthorizationException.
   String toString() {
     var header = 'OAuth authorization error ($error)';
     if (description != null) {
       header = '$header: $description';
     } else if (uri != null) {
       header = '$header: $uri';
     }
     return '$header.';
   }
 }