Make LockFile immutable.

test/lock_file_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 lock_file_test;
 
 import 'dart:async';
 
 import 'package:pub/src/lock_file.dart';
 import 'package:pub/src/package.dart';
@@ skipping 177 matching lines @@
 packages:
   foo:
     bonus: not used
     version: 1.2.3
     source: mock
     description: foo desc
 ''', sources);
       });
     });
 
-    group('serialize()', () {
-      var lockfile;
-      setUp(() {
-        lockfile = new LockFile.empty();
-      });
+    test('serialize() dumps the lockfile to YAML', () {
+      var lockfile = new LockFile([
+        new PackageId(
+          'foo', mockSource.name, new Version.parse('1.2.3'), 'foo desc'),
+        new PackageId(
+          'bar', mockSource.name, new Version.parse('3.2.1'), 'bar desc')
+      ], sources);
 
-      test('dumps the lockfile to YAML', () {
-        lockfile.packages['foo'] = new PackageId(
-            'foo', mockSource.name, new Version.parse('1.2.3'), 'foo desc');
-        lockfile.packages['bar'] = new PackageId(
-            'bar', mockSource.name, new Version.parse('3.2.1'), 'bar desc');
-
-        expect(loadYaml(lockfile.serialize(null, sources)), equals({
-          'packages': {
-            'foo': {
-              'version': '1.2.3',
-              'source': 'mock',
-              'description': 'foo desc'
-            },
-            'bar': {
-              'version': '3.2.1',
-              'source': 'mock',
-              'description': 'bar desc'
-            }
+      expect(loadYaml(lockfile.serialize(null, sources)), equals({
+        'packages': {
+          'foo': {
+            'version': '1.2.3',
+            'source': 'mock',
+            'description': 'foo desc'
+          },
+          'bar': {
+            'version': '3.2.1',
+            'source': 'mock',
+            'description': 'bar desc'
           }
-        }));
-      });
+        }
+      }));
     });
   });
 }