Thread the weak persistent handle free-list through the raw_ field.

Thread the weak persistent handle free-list through the raw_ field.

As free-list pointers are either the address of the next free handle
or NULL, raw_ fields should conveniently appear as a smi and be
ignored during weak reference processing.  Ordinary persistent handles
already reuse the raw_ field for this purpose and rely on confusing
free-list links with tagged smi values.

Of course, nothing strictly requires that the addresses of handles be
aligned in just the right way.  So, just in case, new asserts check
that raw_ field values are not confusable with tagged heap addresses.

Committed: https://code.google.com/p/dart/source/detail?r=4423

[siva, 2012-02-17 23:02:12]

lgtm