Reapply r3590 with frog fixes

Reapply r3590 with frog fixes

Committed: https://code.google.com/p/dart/source/detail?r=3599

[jimhug, 2012-01-25 15:33:22]

TBR=sra

This fixes the bug that forced Stephen to revert r3590 last night and includes
his changes to ensure the fix will hold.

I also uncovered a significant type mistmatch issue between the isolate code in
frog which wanted to pass a JSON array to postMessage and the Worker.postMessage
method that wouldn't accept it.  This mismatch has been present for a while and
was the reason that several isolate tests were failing in checked mode.  I
"fixed" the issue for now by manually editing the generated dom_frog file and
changing these methods to accept var.  I'm sure this isn't the right long-term
answer, but this greatly increases our isolate test coverage today.