Make LockFile immutable.

test/version_solver_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_upgrade_test;
 
 import 'dart:async';
 
 import 'package:pub/src/lock_file.dart';
 import 'package:pub/src/log.dart' as log;
@@ skipping 1148 matching lines @@
     if (result != null) {
       var newResult = {};
       result.forEach((description, version) {
         var id = parseSpec(description, version);
         newResult[id.name] = id;
       });
       result = newResult;
     }
 
     // Parse the lockfile.
-    var realLockFile = new LockFile.empty();
-    if (lockfile != null) {
-      lockfile.forEach((name, version) {
-        version = new Version.parse(version);
-        realLockFile.packages[name] =
-            new PackageId(name, source1.name, version, name);
-      });
+    var realLockFile;
+    if (lockfile == null) {
+      realLockFile = new LockFile.empty(cache.sources);
+    } else {
+      realLockFile = new LockFile(lockfile.keys.map((name) {
+        var version = new Version.parse(lockfile[name]);
+        return new PackageId(name, source1.name, version, name);
+      }), cache.sources);
     }
 
     // Resolve the versions.
     log.verbosity = log.Verbosity.NONE;
     var future = resolveVersions(
         downgrade ? SolveType.DOWNGRADE : SolveType.GET,
         cache.sources, root, lockFile: realLockFile);
 
     var matcher;
     if (result != null) {
@@ skipping 344 matching lines @@
   }
 
   var source = "mock1";
   if (match[7] != null) {
     source = match[7];
     if (source == "root") source = null;
   }
 
   return new PackageId(name, source, parsedVersion, description);
 }