Make LockFile immutable.

lib/src/solver/version_solver.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.solver.version_solver;
 
 import 'dart:async';
 import "dart:convert";
 
 import 'package:pub_semver/pub_semver.dart';
@@ skipping 14 matching lines @@
 /// that those dependencies place on each other and the requirements imposed by
 /// [lockFile].
 ///
 /// If [useLatest] is given, then only the latest versions of the referenced
 /// packages will be used. This is for forcing an upgrade to one or more
 /// packages.
 ///
 /// If [upgradeAll] is true, the contents of [lockFile] are ignored.
 Future<SolveResult> resolveVersions(SolveType type, SourceRegistry sources,
     Package root, {LockFile lockFile, List<String> useLatest}) {
-  if (lockFile == null) lockFile = new LockFile.empty();
+  if (lockFile == null) lockFile = new LockFile.empty(sources);
   if (useLatest == null) useLatest = [];
 
   return log.progress('Resolving dependencies', () {
     return new BacktrackingSolver(type, sources, root, lockFile, useLatest)
         .solve();
   });
 }
 
 /// The result of a version resolution.
 class SolveResult {
@@ skipping 432 matching lines @@
 
   DependencyNotFoundException(String package, this._innerException,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   /// The failure isn't because of the version of description of the package,
   /// it's the package itself that can't be found, so just show the name and no
   /// descriptive details.
   String _describeDependency(PackageDep dep) => "";
 }