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First steps towards general pretenuring support. * Generalize Heap::Top/EndAddress. * Add testing flag to exercise new code paths. * Also update the slow-case runtime calls, to ensure a fresh block will be allocated in old. Next steps are general invalidation of the generated code and adding a policy that doesn't blow up the store buffers), and freelist allocation. Review URL: https://codereview.chromium.org//578443003 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@40530 260f80e4-7a28-3924-810f-c04153c831b5