Add stack chain support to pub.

sdk/lib/_internal/pub/lib/src/solver/version_queue.dart

 // Copyright (c) 2013, 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.solver.version_queue;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 
 import '../package.dart';
-import 'backtracking_solver.dart';
 
 /// A function that asynchronously returns a sequence of package IDs.
 typedef Future<Iterable<PackageId>> PackageIdGenerator();
 
 /// A prioritized, asynchronous queue of the possible versions that can be
 /// selected for one package.
 ///
 /// If there is a locked version, that comes first, followed by other versions
 /// in descending order. This avoids requesting the list of versions until
 /// needed (i.e. after any locked version has been consumed) to avoid unneeded
@@ skipping 61 matching lines @@
   }
 
   /// Determines the list of allowed versions matching its constraint and places
   /// them in [_allowed].
   Future _calculateAllowed() {
     return _allowedGenerator().then((allowed) {
       _allowed = new Queue<PackageId>.from(allowed);
     });
   }
 }