tree-ir bugfix: Count unseen uses instead of seen uses.

tree-ir bugfix: Count unseen uses instead of seen uses.

Keeping track of the number of "seen uses" of a variable is too fragile
since it needs bookkeeping whenever we merge two variable uses.

I've changed it to track the number of unseen uses instead. The
bookkeeping is simpler because we aren't going to transform an unseen
use.

BUG=
R=kmillikin@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45207

[asgerf, 2015-04-16 12:03:54]

asgerf@google.com changed reviewers:
+ kmillikin@google.com

[asgerf, 2015-04-16 12:03:55]

https://codereview.chromium.org/1080343003/diff/1/pkg/compiler/lib/src/tree_i...
File pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart (left):

https://codereview.chromium.org/1080343003/diff/1/pkg/compiler/lib/src/tree_i...
pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart:218:
--seenUses[node.variable]; // Do not count the use we just destroyed.
I knew this was a bad idea and somehow I did it anyway :(

https://codereview.chromium.org/1080343003/diff/1/pkg/compiler/lib/src/tree_i...
File pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart (right):

https://codereview.chromium.org/1080343003/diff/1/pkg/compiler/lib/src/tree_i...
pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart:903: combined
= new VariableUse(use1.variable);
FYI this is where the bookkeeping was missing.

Updating seenUses here would work, but clearly the issue would come back to
haunt us again.

[Kevin Millikin (Google), 2015-04-16 12:50:38]

lgtm

[asgerf, 2015-04-16 13:03:19]

Committed patchset #1 (id:1) manually as 45207 (presubmit successful).