Integrate leak_tracker

[Solido]

Leak Tracker is already integrated with Flutter but, counterintuitively, not with pure dart project where it could be important consolidation for long running programs.

Server-side or complexe desktops apps would be first citizen.

@polina-c is already on the subject and may need some short feature from the core team to deliver this awesome tool.

dart-lang/leak_tracker#257

Thanks!

[polina-c]

This feature does not need any changes from leak_tracker side. Just integration with the function test, similar to what was done for testWidget.

And the question is: does Dash team have bandwidth for it in the nearest future or external contributions is only way to make it happen?