Wait for concurrent sweeper in vm/cc/PrintJSON.
Wait for concurrent sweeper in vm/cc/PrintJSON.

The test uses Heap::IterateObjects, which may visit garbage; see https://code.google.com/p/dart/issues/detail?id=21620

BUG=dart:21620
R=iposva@google.com

Review URL: https://codereview.chromium.org//729603004

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@41755 260f80e4-7a28-3924-810f-c04153c831b5

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