MemorySanitizer support.

Enables building (including generating snapshot) and running simple scripts under MemorySanitizer.
Not all tests pass yet.

R=asiva@google.com

Review URL: https://codereview.chromium.org//816123002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42561 260f80e4-7a28-3924-810f-c04153c831b5