Avoid verifying heap while sweeper is running.

Avoid verifying heap while sweeper is running.

Verifier cannot handle a combination of dead+unswept objects and dead+swept objects.
* Verify before marking, before sweeping, and after a non-concurrent sweep.
* Disable pre/post-scavenge verification when concurrently sweeping.
* On shutdown, wait until concurrent GC tasks finish before verifying.

R=iposva@google.com
TBR=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40946

[koda, 2014-10-06 23:16:46]

Submitting TBR to allow another attempt at enabling --concurrent_sweep (in DEBUG
mode, we always verify heap at shutdown).

[Ivan Posva, 2014-10-06 23:20:07]

LGTM -ip

[koda, 2014-10-06 23:20:28]

Committed patchset #2 (id:20001) manually as r40946 (presubmit successful).