Overrides on FlutterError.onError does not take effect.

[ye-yu]

Describe the bug
Overrides on FlutterError.onError does not take effect. Overriding this error is useful for ignoring unnecessary errors.

To Reproduce

1. Create a widget that throws error.
2. Create a test that ignores this specific error using FlutterError.onError.

   FlutterError.onError = (error) {
     if (error is SpecificError) return;
     FlutterError.presentError(error);
   }
   addTearDown(() => FlutterError.onError = FlutterError.presentError;

3. Error is not ignored as expected.

Expected behavior
FlutterError.onError should be called. If the condition matched, error is not reported.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Should I create a PR to remove this line or replace this with FlutterError.onError?

https://github.com/fzyzcjy/flutter_convenient_test/blob/master/packages/convenient_test_dev/lib/src/functions/widget_tester.dart#L191

[welcome]

Hi! Thanks for opening your first issue here! 😄

[fzyzcjy]

Hmm, looks like we can directly do

convenientTestRunAsyncEnhancedExceptionChecker = (e) {
  if (e is SpecificError) return;
  expect(e, isNull);
}

to ensure we ignore the specific error.