Make LockFile immutable.

lib/src/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 pub.entrypoint;
 
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 import 'package:barback/barback.dart';
@@ skipping 448 matching lines @@
       if (result != null) {
         var packages = new Map.fromIterable(result.packages,
             key: (id) => id.name,
             value: (id) {
           if (id.name == root.name) return root;
 
           return new Package(result.pubspecs[id.name],
               cache.sources[id.source].getDirectory(id));
         });
 
-        return new PackageGraph(this, new LockFile(result.packages), packages);
+        return new PackageGraph(
+            this,
+            new LockFile(result.packages, cache.sources),
+            packages);
       }
 
       await ensureLockFileIsUpToDate();
       var packages = new Map.fromIterable(lockFile.packages.values,
           key: (id) => id.name,
           value: (id) {
         var dir = cache.sources[id.source].getDirectory(id);
         return new Package.load(id.name, dir, cache.sources);
       });
       packages[root.name] = root;
       return new PackageGraph(this, lockFile, packages);
     }, fine: true);
 
     _packageGraph = graph;
     return graph;
   }
 
   /// Saves a list of concrete package versions to the `pubspec.lock` file.
   void _saveLockFile(List<PackageId> packageIds) {
-    _lockFile = new LockFile(packageIds);
+    _lockFile = new LockFile(packageIds, cache.sources);
     var lockFilePath = root.path('pubspec.lock');
-    writeTextFile(lockFilePath, _lockFile.serialize(root.dir, cache.sources));
+    writeTextFile(lockFilePath, _lockFile.serialize(root.dir));
   }
 
   /// Creates a self-referential symlink in the `packages` directory that allows
   /// a package to import its own files using `package:`.
   void _linkSelf() {
     var linkPath = path.join(packagesDir, root.name);
     // Create the symlink if it doesn't exist.
     if (entryExists(linkPath)) return;
     ensureDir(packagesDir);
     createPackageSymlink(root.name, root.dir, linkPath,
@@ skipping 46 matching lines @@
   /// If [packageSymlinks] is true, creates a symlink to the "packages"
   /// directory in [dir].
   ///
   /// Otherwise, deletes a "packages" directories in [dir] if one exists.
   void _linkOrDeleteSecondaryPackageDir(String dir) {
     var symlink = path.join(dir, 'packages');
     if (entryExists(symlink)) deleteEntry(symlink);
     if (_packageSymlinks) createSymlink(packagesDir, symlink, relative: true);
   }
 }