Use backtracking when solving dependency constraints.

utils/tests/pub/update/hosted/update_removed_constraints_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("updates dependencies whose constraints have been removed", () {
     servePackages([
       packageMap("foo", "1.0.0", [dependencyMap("shared-dep")]),
       packageMap("bar", "1.0.0", [dependencyMap("shared-dep", "<2.0.0")]),
       packageMap("shared-dep", "1.0.0"),
       packageMap("shared-dep", "2.0.0")
     ]);
 
     d.appDir([dependencyMap("foo"), dependencyMap("bar")]).create();
 
@@ skipping 11 matching lines @@
     schedulePub(args: ['update'],
         output: new RegExp(r"Dependencies updated!$"));
 
     d.packagesDir({
       "foo": "1.0.0",
       "bar": null,
       "shared-dep": "2.0.0"
     }).validate();
   });
 }