Cleanup handles more efficiently, ensures we don't have to iterate over them

Cleanup handles more efficiently, ensures we don't have to iterate over them
while visiting objects during GC.
- Add handle scopes during class finialization
- Add handle scopes to different stages of the compiler
- Ensure that we iterate only to the top of scoped handles while visiting objects
Committed: https://code.google.com/p/dart/source/detail?r=16683

[Ivan Posva, 2013-01-06 19:46:06]

LGTM -ip