Use backtracking when solving dependency constraints.

utils/tests/pub/update/git/do_not_update_if_unneeded_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("doesn't update one locked Git package's dependencies if it's "
       "not necessary", () {
     ensureGit();
 
     d.git('foo.git', [
       d.libDir('foo'),
       d.libPubspec("foo", "1.0.0", deps: [{"git": "../foo-dep.git"}])
     ]).create();
 
     d.git('foo-dep.git', [
@@ skipping 31 matching lines @@
     d.dir(packagesPath, [
       d.dir('foo', [
         d.file('foo.dart', 'main() => "foo 2";')
       ]),
       d.dir('foo-dep', [
         d.file('foo-dep.dart', 'main() => "foo-dep";')
       ]),
     ]).validate();
   });
 }