test [nfc]: Factor out checkNoErrorDialog helper

With zulip#996, these tests will have to start checking for separate
per-platform flavors of alert dialog. Best if they all do so through
this one codepath.

Author: chrisbobbe

test/widgets/app_test.dart

@@ -280,14 +280,14 @@ void main() {
       check(ZulipApp.ready).value.isFalse();
       await tester.pump();
       check(findSnackBarByText(message).evaluate()).isEmpty();
-      check(find.byType(AlertDialog).evaluate()).isEmpty();
+      checkNoErrorDialog(tester);
 
       check(ZulipApp.ready).value.isTrue();
       // After app startup, reportErrorToUserBriefly displays a SnackBar.
       reportErrorToUserBriefly(message, details: details);
       await tester.pumpAndSettle();
       check(findSnackBarByText(message).evaluate()).single;
-      check(find.byType(AlertDialog).evaluate()).isEmpty();
+      checkNoErrorDialog(tester);
 
       // Open the error details dialog.
       await tester.tap(find.text('Details'));

test/widgets/compose_box_test.dart

@@ -427,8 +427,7 @@ void main() {
       await setupAndTapSend(tester, prepareResponse: (int messageId) {
         connection.prepare(json: SendMessageResult(id: messageId).toJson());
       });
-      final errorDialogs = tester.widgetList(find.byType(AlertDialog));
-      check(errorDialogs).isEmpty();
+      checkNoErrorDialog(tester);
     });
 
     testWidgets('ZulipApiException', (tester) async {
@@ -497,8 +496,7 @@ void main() {
         check(call.allowMultiple).equals(true);
         check(call.type).equals(FileType.media);
 
-        final errorDialogs = tester.widgetList(find.byType(AlertDialog));
-        check(errorDialogs).isEmpty();
+        checkNoErrorDialog(tester);
 
         check(controller!.content.text)
           .equals('see image: [Uploading image.jpg…]()\n\n');
@@ -557,8 +555,7 @@ void main() {
         check(call.source).equals(ImageSource.camera);
         check(call.requestFullMetadata).equals(false);
 
-        final errorDialogs = tester.widgetList(find.byType(AlertDialog));
-        check(errorDialogs).isEmpty();
+        checkNoErrorDialog(tester);
 
         check(controller!.content.text)
           .equals('see image: [Uploading image.jpg…]()\n\n');

test/widgets/dialog_checks.dart

@@ -1,4 +1,6 @@
+import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter_checks/flutter_checks.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:zulip/widgets/dialog.dart';
 
@@ -26,6 +28,11 @@ Widget checkErrorDialog(WidgetTester tester, {
       matching: find.widgetWithText(TextButton, 'OK')));
 }
 
+// TODO(#996) update this to check for per-platform flavors of alert dialog
+void checkNoErrorDialog(WidgetTester tester) {
+  check(find.byType(AlertDialog)).findsNothing();
+}
+
 /// In a widget test, check that [showSuggestedActionDialog] was called
 /// with the right text.
 ///