Disable an expensive GC assert unless heap verification also enabled.

Disable an expensive GC assert unless heap verification also enabled.

This assert verifies every pointer being pushed on the mark stack as
belonging to old-space.  Unfortunately, it turns out that checking for
containment in old-space domainates the running time of programs that
frequently invoke an old-space garbage collection.

Committed: https://code.google.com/p/dart/source/detail?r=11209

[cshapiro, 2012-08-23 04:23:52]

TBR=iposva

[Ivan Posva, 2012-08-23 06:02:59]

LGTM -ip