add a comment to the generated lock file

utils/pub/lock_file.dart

Index: utils/pub/lock_file.dart
===================================================================
--- utils/pub/lock_file.dart	(revision 17925)
+++ utils/pub/lock_file.dart	(working copy)
@@ -26,7 +26,6 @@
     var packages = <String, PackageId>{};
 
     if (contents.trim() == '') return new LockFile.empty();
-
     var parsed = loadYaml(contents);
 
     if (parsed.containsKey('packages')) {
@@ -85,6 +84,9 @@
 
     // 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})}
+        ''';
   }
 }