Deoptimize the world when single stepping.

Deoptimize the world when single stepping.

It is possible that we may be stepping into an optimized frame
otherwise.  In fact, the current frame may be optimized if the isolate
was interrupted instead of hitting a breakpoint.

BUG=
R=hausner@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42348

[turnidge, 2014-12-12 21:29:47]

turnidge@google.com changed reviewers:
+ hausner@google.com

[hausner, 2014-12-12 22:00:45]

lgtm

[turnidge, 2014-12-12 22:21:32]

Committed patchset #3 (id:40001) manually as 42348 (presubmit successful).