- Always write a Null Object for Code objects

- Always write a Null Object for Code objects
- Do not populate the FunctionsCache array as it causes the function cache
  to become inconsistent if we execute some dart code before capturing a
  snapshot.
- Allow preallocation of objects even when generating snapshot and remove the
  flag preallocate_objects_called_ (use a null test instead).
Committed: https://code.google.com/p/dart/source/detail?r=4915

[Ivan Posva, 2012-03-03 00:48:59]

LGTM -ip