[espresso] (feature request) doesNotExist assertion for widgets

### 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](https://github.com/flutter/flutter/blob/master/docs/ecosystem/testing/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:

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