Use backtracking when solving dependency constraints.

utils/tests/pub/install/hosted/stay_locked_if_compatible_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 a locked pub server package with a new "
       "compatible constraint", () {
     servePackages([packageMap("foo", "1.0.0")]);
 
     d.appDir([dependencyMap("foo")]).create();
 
     schedulePub(args: ['install'],
         output: new RegExp(r"Dependencies installed!$"));
 
     d.packagesDir({"foo": "1.0.0"}).validate();
 
     servePackages([packageMap("foo", "1.0.1")]);
 
     d.appDir([dependencyMap("foo", ">=1.0.0")]).create();
 
     schedulePub(args: ['install'],
         output: new RegExp(r"Dependencies installed!$"));
 
     d.packagesDir({"foo": "1.0.0"}).validate();
   });
 }