add a comment to the generated lock file

utils/tests/pub/lock_file_test.dart

Index: utils/tests/pub/lock_file_test.dart
===================================================================
--- utils/tests/pub/lock_file_test.dart	(revision 17925)
+++ utils/tests/pub/lock_file_test.dart	(working copy)
@@ -162,8 +162,8 @@
             'foo', mockSource, new Version.parse('1.2.3'), 'foo desc');
         lockfile.packages['bar'] = new PackageId(
             'foo', mockSource, new Version.parse('3.2.1'), 'bar desc');
-
-        expect(loadYaml(lockfile.serialize()), equals({
+        
+        expect(loadYaml(lockfile.serialize().replaceAll(LockFile.comment, '')), equals({

[Bob Nystrom, 2013/01/31 22:17:44]

This shouldn't be needed. If it is, there's a bug in YAML! :)

[keertip, 2013/01/31 22:23:39]

If the comment is not removed, loadYaml fails and returns null

On 2013/01/31 22:17:44, Bob Nystrom wrote:

[keertip, 2013/01/31 22:31:40]

Guess that was because the comment should be on a separate line! Will make the
changes.
 
On 2013/01/31 22:23:39, keertip wrote:

[Bob Nystrom, 2013/01/31 22:33:48]

That was my hunch too. :)

           'packages': {
             'foo': {
               'version': '1.2.3',