The previous algorithm would visit each pointer in the heap and verify it
without regard for whether the pointer had already been visited.
The new algorithm computes the set of allocated objects and verifies each
object in the set. In a second pass, each pointer is visited and tested
for membership in the set.
BUG=2606
Review URL: https://chromiumcodereview.appspot.com//10696029
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@9532 260f80e4-7a28-3924-810f-c04153c831b5
cshapiro@google.com