Finder byKey(Key key, { bool skipOffstage: true })

Finds widgets by searching for one with a particular Key.

Example:

expect(tester, hasWidget(find.byKey(backKey)));

If the skipOffstage argument is true (the default), then this skips nodes that are Offstage or that are from inactive Routes.

Source 

Finder byKey(Key key, { bool skipOffstage: true }) => new _KeyFinder(key, skipOffstage: skipOffstage);