add a comment to the generated lock file

utils/pub/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 'package.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
 import '../../pkg/yaml/lib/yaml.dart';
 
 /// A parsed and validated `pubspec.lock` file.
 class LockFile {
   /// The packages this lockfile pins.
   Map<String, PackageId> packages;
 
   LockFile._(this.packages);
 
   LockFile.empty()
     : packages = <String, PackageId>{};
 
   /// Parses the lockfile whose text is [contents].
   factory LockFile.parse(String contents, SourceRegistry sources) {
     var packages = <String, PackageId>{};
 
     if (contents.trim() == '') return new LockFile.empty();
-
     var parsed = loadYaml(contents);
 
     if (parsed.containsKey('packages')) {
       var packageEntries = parsed['packages'];
 
       packageEntries.forEach((name, spec) {
         // Parse the version.
         if (!spec.containsKey('version')) {
           throw new FormatException('Package $name is missing a version.');
         }
@@ skipping 38 matching lines @@
     packages.forEach((name, id) {
       packagesObj[name] = {
         'version': id.version.toString(),
         'source': id.source.name,
         'description': id.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 json.stringify({'packages': packagesObj});
+    return '''
+        # Generated by pub. See: http://pub.dartlang.org/doc/glossary.html#lockfile
+         ${json.stringify({'packages': packagesObj})}
+        ''';
   }
 }