Write-barrier verification on ARM.

Add and use methods on Assembler for writing and verifying object fields.
Enable tests in verified_mem mode for simarm.
Also generalize and use InitializeFieldsNoBarrierUnrolled in one more place.

R=iposva@google.com

Review URL: https://codereview.chromium.org//730343005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42208 260f80e4-7a28-3924-810f-c04153c831b5