Fix broken pub tests.

sdk/lib/_internal/pub/test/cache/add/unexpected_arguments_test.dart

 // Copyright (c) 2014, 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 pub_tests;
 
 import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('fails if there are extra arguments', () {
     schedulePub(args: ["cache", "add", "foo", "bar", "baz"],
         error: """
             Unexpected arguments "bar" and "baz".
             
             Usage: pub cache add <package> [--version <constraint>] [--all]
             -h, --help       Print usage information for this command.
                 --all        Install all matching versions.
             -v, --version    Version constraint.
+
+            See http://dartlang.org/tools/pub/cmd/pub-cache.html for detailed documentation.
             """,
         exitCode: exit_codes.USAGE);
   });
 }