b1634f7c30
Prepares for multiple threads referencing the same isolate (in particular, the mutator and sweeper). R=asiva@google.com Review URL: https://codereview.chromium.org//999983004 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@44762 260f80e4-7a28-3924-810f-c04153c831b5