Reapply "Refactor type propagation."

Reapply "Refactor type propagation."

This reapplies (a corrected version of ) commit 6599 which had been reverted in commit 6600.

Original CL: https://chromiumcodereview.appspot.com//10098001

Committed: https://code.google.com/p/dart/source/detail?r=6601

[floitsch, 2012-04-16 20:13:02]

Patchset 1 is the original (unfixed) CL.
Patchset 2 applies the fix.

Submitting TBR since the change is absolutely minor and the original CL was
already TBRed.

[floitsch, 2012-04-16 20:13:14]

On 2012/04/16 20:13:02, floitsch wrote:
> Patchset 1 is the original (unfixed) CL.
> Patchset 2 applies the fix.
> 
> Submitting TBR since the change is absolutely minor and the original CL was
> already TBRed.

s/TBRed/LGTMed.

[kasperl, 2012-04-17 05:32:51]

LGTM!