Improve the reporting of scavenger and free list reports.

Improve the reporting of scavenger and free list reports.

The scavenger now reports survival and promotion statistics.

The statistics for the large free list are now reported.  Previously,
only information about the small free list was printed.  Each report
indicates whether the data is from before or after a garbage collection,
each row of a report is preceded by the free list type (small or large),
and cumulative bytes are reported for the free list type.

Committed: https://code.google.com/p/dart/source/detail?r=16074

[hausner, 2012-12-12 22:57:50]

lgtm