Support verified heap pointer writes on ia32.

With --verified_mem, use VerifiedMemory to duplicate all pointer writes in the heap, and verify that no unaccounted writes occurred.

R=iposva@google.com

Review URL: https://codereview.chromium.org//711833002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41700 260f80e4-7a28-3924-810f-c04153c831b5