Use backtracking when solving dependency constraints.

utils/tests/pub/install/git/check_out_with_trailing_slash_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 '../../../../../pkg/scheduled_test/lib/scheduled_test.dart';
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
+  initConfig();
   group("(regression)", () {
     integration('checks out a package from Git with a trailing slash', () {
       ensureGit();
 
       d.git('foo.git', [
         d.libDir('foo'),
         d.libPubspec('foo', '1.0.0')
       ]).create();
 
       d.appDir([{"git": "../foo.git/"}]).create();
 
       schedulePub(args: ['install'],
           output: new RegExp(r"Dependencies installed!$"));
 
       d.dir(cachePath, [
         d.dir('git', [
           d.dir('cache', [d.gitPackageRepoCacheDir('foo')]),
           d.gitPackageRevisionCacheDir('foo')
         ])
       ]).validate();
 
       d.dir(packagesPath, [
         d.dir('foo', [
           d.file('foo.dart', 'main() => "foo";')
         ])
       ]).validate();
     });
   });
 }