Add a package for authenticating via OAuth2.

pkg/oauth2/lib/src/authorization_exception.dart

Index: pkg/oauth2/lib/src/authorization_exception.dart
diff --git a/pkg/oauth2/lib/src/authorization_exception.dart b/pkg/oauth2/lib/src/authorization_exception.dart
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+++ b/pkg/oauth2/lib/src/authorization_exception.dart
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+// 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';
+
+/// 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.';
+  }
+}