Investigate why leak tracker did not catch not disposed picture.

[polina-c]

This leak was caused by not disposed disposable, but leak tracker did not catch it: flutter/flutter#153678

This is a drafty PR that enables leak tracking for web and documents how to run it for running applications. I will send set of PRs to land the change.

But it is unclear why leak tracker did not catch the leak in both testWidgets and in the running application. We need either to fix the issue, or document as a limitation of leak tracker.