Fix race condition and spelling error in metadata reader

2 separate fixes. See individual commit messages for more details.

1) Race condition revealed by http://roslyn.codeplex.com/workitem/299
2) Spelling error in rarely used pre-release API name




Not sure if it's worth it, but it seems like you might be able to force the issue by wrapping the FileStream with your own custom stream that overrides Flush to use synchronization such that it won't complete until you let it.  Then you should be able to queue two work items that access the problematic functionality and track in your override of Flush whether the calls occurred concurrently.