Fix reference counting bug in StatementRewriter.

Fix reference counting bugs in StatementRewriter.

BUG=
R=kmillikin@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44231

[asgerf, 2015-03-04 10:12:47]

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

[Kevin Millikin (Google), 2015-03-04 10:35:30]

lgtm

https://codereview.chromium.org/970853006/diff/1/pkg/compiler/lib/src/tree_ir...
File pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart (right):

https://codereview.chromium.org/970853006/diff/1/pkg/compiler/lib/src/tree_ir...
pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart:527:
--s.variable.writeCount;
I guess we should be consistent within this file about whether we use pre- or
post-decrement.

I tend to use pre-increment and decrement when I don't care about the value
because in English (and Lisp) the verb normally comes before the object:
"decrement writeCount".  But I don't really care too much one way or the other.

[asgerf, 2015-03-04 13:32:10]

Committed patchset #2 (id:20001) manually as 44231 (presubmit successful).