Use backtracking when solving dependency constraints.

utils/pub/entrypoint.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 entrypoint;
 
 import 'dart:async';
 
 import '../../pkg/pathos/lib/path.dart' as path;
 
 import 'io.dart';
 import 'lock_file.dart';
 import 'log.dart' as log;
 import 'package.dart';
 import 'pubspec.dart';
 import 'sdk.dart' as sdk;
 import 'system_cache.dart';
 import 'utils.dart';
 import 'version.dart';
-import 'version_solver.dart';
+import 'version_solver2.dart';
 
 /// Pub operates over a directed graph of dependencies that starts at a root
 /// "entrypoint" package. This is typically the package where the current
 /// working directory is located. An entrypoint knows the [root] package it is
 /// associated with and is responsible for managing the "packages" directory
 /// for it.
 ///
 /// That directory contains symlinks to all packages used by an app. These links
 /// point either to the [SystemCache] or to some other location on the local
 /// filesystem.
@@ skipping 269 matching lines @@
       var symlink = path.join(dir, 'packages');
       if (fileExists(symlink)) {
         deleteFile(symlink);
       } else if (dirExists(symlink)) {
         deleteDir(symlink);
       }
       return createSymlink(packagesDir, symlink, relative: true);
     });
   }
 }