[espresso] (feature request) doesNotExist assertion for widgets

[FirentisTFW]

Use case

I'm trying to add an Android integration test to one of the packages, I use Espresso for it, like suggested in Plugin-Tests.md. I need to check if a widget is not rendered on the screen, after that I'll perform an action, and then verify the existence of a widget again (this time it should be visible).

In native Espresso library we can do that with ViewAssertions.doesNotExist, but there's no equivalent in Espresso for Flutter - we only have isExisting here.

Proposal

I'd suggest adding another static function (doesNotExist) to FlutterMatchers class. This function would return a proper matcher that would check if a given Flutter widget exists. If it does, it should throw an exception.

Example code:

WidgetInteraction playButton = onFlutterWidget(withValueKey("Play"));
playButton.check(matches(doesNotExist()));

[jmagman]

I'd suggest adding another static function (doesNotExist) to FlutterMatchers class.

Sounds like a good idea! We would happily review such a pull request to add it.