Fix unittest in checked mode

Fix unittest in checked mode
(partial revert of https://codereview.chromium.org/12328090)

Committed: https://code.google.com/p/dart/source/detail?r=19068

[Siggi Cherem (dart-lang), 2013-02-26 18:45:32]

I'm doing a quick  revert of these assertions and we can investigate later.
Turns out that unittest_test fails in checked mode (I only ran them in
production mode initially)

[Siggi Cherem (dart-lang), 2013-02-26 18:45:47]

Committed patchset #1 manually as r19068 (presubmit successful).

[gram, 2013-02-26 18:59:10]

lgtm