Decrease size of test of write barrier verification.

It was close to timing out on Linux, and failing on Windows, likely due to OOM.

TBR=iposva@google.com

Review URL: https://codereview.chromium.org//723893002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41702 260f80e4-7a28-3924-810f-c04153c831b5