Prune variables that are not live at block entry from the environment.

Prune variables that are not live at block entry from the environment.

If a variable is not live at a block entry, replace it
with the null-constant in the environment.  This can occur
when there is a partially dead variable stores.

Fewer environment uses mean shorter live ranges and better register
allocation.

R=srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=24939

[srdjan, 2013-07-11 17:03:54]

lgtm

[Florian Schneider, 2013-07-12 08:27:39]

Committed patchset #1 manually as r24939 (presubmit successful).