Create a LoggingAllocator

[timsneath]

A logging allocator class would behave as follows:

• For release builds, it would simply wrap the existing calloc / free model with no significant overhead
• For debug builds, it would track the number and size of allocations it had made, along with as much metadata as can be captured on the source of the allocation. On freeing memory, it would remove the equivalent entry from the tracking log. At any time, it should be possible to retrieve the log.

The logging allocator would help detect memory leaks during test runs, by validating that all memory allocated had been freed by the end of the run.

Idea inspired by @dcharkes.

[halildurmus]

Yes please!

[timsneath]

Potential solution: dart-lang/leak_tracker#42