Sort pubspec.lock packages.

sdk/lib/_internal/pub/lib/src/lock_file.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;
 
 import 'dart:json' as json;
+import 'dart:collection';
 
 import 'package:yaml/yaml.dart';
 
 import 'io.dart';
 import 'package.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
 
 /// A parsed and validated `pubspec.lock` file.
@@ skipping 63 matching lines @@
 
         packages[name] = id;
       });
     }
 
     return new LockFile._(packages);
   }
 
   /// Returns the serialized YAML text of the lock file.
   String serialize() {
-    var packagesObj = <String, Map>{};
-    packages.forEach((name, id) {
+    var packagesObj = new LinkedHashMap<String, Map>();
+
+    // Sort the packages by name.
+    var sortedKeys = packages.keys.toList();
+    sortedKeys.sort();
+    sortedKeys.forEach((name) {
       packagesObj[name] = {
-        'version': id.version.toString(),
-        'source': id.source.name,
-        'description': id.description
+        'version': packages[name].version.toString(),
+        'source': packages[name].source.name,
+        'description': packages[name].description
       };
     });
 
     // TODO(nweiz): Serialize using the YAML library once it supports
     // serialization. For now, we use JSON, since it's a subset of YAML anyway.
     return
         '# Generated by pub\n'
         '# See http://pub.dartlang.org/doc/glossary.html#lockfile\n'
         '\n'
         '${json.stringify({'packages': packagesObj})}\n';
   }
 }