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




I definitely think it is worth the extra effort to force the blocking to happen in the test. The current test (if I understand it correctly) is NOT guaranteed to reliably reproduce the problem, and therefore is not effective at preventing a regression in the future.