Add assertion that snackbars have a place to go (flutter#103006)

Author: Piinks

packages/flutter/lib/src/material/scaffold.dart

@@ -265,6 +265,11 @@ class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProvide
   /// ** See code in examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.0.dart **
   /// {@end-tool}
   ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackBar) {
+    assert(
+      _scaffolds.isNotEmpty,
+      'ScaffoldMessenger.showSnackBar was called, but there are currently no '
+      'descendant Scaffolds to present to.',
+    );
     _snackBarController ??= SnackBar.createAnimationController(vsync: this)
       ..addStatusListener(_handleSnackBarStatusChanged);
     if (_snackBars.isEmpty) {

packages/flutter/test/material/snack_bar_test.dart

@@ -2090,6 +2090,34 @@ void main() {
     await tester.pump(const Duration(milliseconds: 750));
     await expectLater(find.byType(MaterialApp), matchesGoldenFile('snack_bar.goldenTest.backdropFilter.png'));
   });
+
+  testWidgets('ScaffoldMessenger will alert for snackbars that cannot be presented', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/103004
+    await tester.pumpWidget(const MaterialApp(
+      home: Center(),
+    ));
+
+    final ScaffoldMessengerState scaffoldMessengerState = tester.state<ScaffoldMessengerState>(
+      find.byType(ScaffoldMessenger),
+    );
+    expect(
+      () {
+        scaffoldMessengerState.showSnackBar(const SnackBar(
+          content: Text('I am a snack bar.'),
+        ));
+      },
+      throwsA(
+        isA<AssertionError>().having(
+          (AssertionError error) => error.toString(),
+          'description',
+          contains(
+            'ScaffoldMessenger.showSnackBar was called, but there are currently '
+            'no descendant Scaffolds to present to.'
+          )
+        ),
+      ),
+    );
+  });
 }
 
 /// Start test for "SnackBar dismiss test".