Some fixes to GC heuristics.
Some fixes to GC heuristics.

The loop intended to do a binary search for the best heap growth size
did not have any variables in it that changed the decision, so it would
make the same decision (up or down) every time around the loop.

R=kasperl@google.com, vegorov@google.com
BUG=

Review-Url: https://codereview.chromium.org/2829853007 .

dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk