Use backtracking when solving dependency constraints.

utils/tests/pub/install/git/require_pubspec_name_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('requires the dependency to have a pubspec with a name '
       'field', () {
     ensureGit();
 
     d.git('foo.git', [
       d.libDir('foo'),
       d.pubspec({})
     ]).create();
 
     d.appDir([{"git": "../foo.git"}]).create();
 
     // TODO(nweiz): clean up this RegExp when either issue 4706 or 4707 is
     // fixed.
     schedulePub(args: ['install'],
         error: new RegExp(r'^Package "foo"' "'" 's pubspec.yaml file is '
             r'missing the required "name" field \(e\.g\. "name: foo"\)\.'),
         exitCode: 1);
   });
 }