Write barrier audit: Verify previous value in generated code.

Always use far jumps in verified mem mode, rather than clutter the code with tests everywhere.

Review URL: https://codereview.chromium.org//721233002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41785 260f80e4-7a28-3924-810f-c04153c831b5
add ContextsChangedEvent

BUG=
R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//732093003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41786 260f80e4-7a28-3924-810f-c04153c831b5
Support verified heap pointer writes on x64.

Port of r41607, r41700, and upcoming change:
https://codereview.chromium.org/721233002/

R=iposva@google.com

Review URL: https://codereview.chromium.org//727623002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41787 260f80e4-7a28-3924-810f-c04153c831b5