Reapply "Avoid "=== true" inside code by calling a function that does this for us."

Reapply "Avoid "=== true" inside code by calling a function that does this for us."

This reapplies a fixed version of commit 7000.
Original CL: https://chromiumcodereview.appspot.com/10140027/

Committed: https://code.google.com/p/dart/source/detail?r=7068

[floitsch, 2012-04-27 11:19:45]

Turns out the self-hosting failure was due to not providing the correct
lib-files. This CL didn't cause the failure.
Applying TBR.

Note: changes to original version can be seen by comparing uploaded Patchset 1
and patchset 2. I added an assert that would have helped diagnose the problem
earlier.