Use backtracking when solving dependency constraints.

utils/tests/pub/update/hosted/fail_gracefully_on_missing_package_test.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 pub_tests;
 
 import 'dart:io';
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
+  initConfig();
   integration('fails gracefully if the package does not exist', () {
     servePackages([]);
 
     d.appDir([dependencyMap("foo", "1.2.3")]).create();
 
     schedulePub(args: ['update'],
         error: new RegExp('Could not find package "foo" at '
                             'http://localhost:'),
         exitCode: 1);
   });
 }