Revert "Remove a dynamic that is no longer necessary (and the TODO for it) (#80294)" (#80326)

This reverts commit 12ec7dc8.

packages/flutter/lib/src/widgets/focus_manager.dart

@@ -55,7 +55,8 @@ enum KeyEventResult {
 ///
 /// Returns a [KeyEventResult] that describes how, and whether, the key event
 /// was handled.
-typedef FocusOnKeyCallback = KeyEventResult Function(FocusNode node, RawKeyEvent event);
+// TODO(gspencergoog): Convert this from dynamic to KeyEventResult once migration is complete.
+typedef FocusOnKeyCallback = dynamic Function(FocusNode node, RawKeyEvent event);
 
 /// An attachment point for a [FocusNode].
 ///
@@ -1673,18 +1674,33 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
     bool handled = false;
     for (final FocusNode node in <FocusNode>[_primaryFocus!, ..._primaryFocus!.ancestors]) {
       if (node.onKey != null) {
-        final KeyEventResult result = node.onKey!(node, event);
-        switch (result) {
-          case KeyEventResult.handled:
+        // TODO(gspencergoog): Convert this from dynamic to KeyEventResult once migration is complete.
+        final dynamic result = node.onKey!(node, event);
+        assert(
+          result is bool || result is KeyEventResult,
+          'Value returned from onKey handler must be a non-null bool or KeyEventResult, not ${result.runtimeType}',
+        );
+        if (result is KeyEventResult) {
+          switch (result) {
+            case KeyEventResult.handled:
+              assert(_focusDebug('Node $node handled key event $event.'));
+              handled = true;
+              break;
+            case KeyEventResult.skipRemainingHandlers:
+              assert(_focusDebug('Node $node stopped key event propagation: $event.'));
+              handled = false;
+              break;
+            case KeyEventResult.ignored:
+              continue;
+          }
+        } else if (result is bool){
+          if (result) {
             assert(_focusDebug('Node $node handled key event $event.'));
             handled = true;
             break;
-          case KeyEventResult.skipRemainingHandlers:
-            assert(_focusDebug('Node $node stopped key event propagation: $event.'));
-            handled = false;
-            break;
-          case KeyEventResult.ignored:
+          } else {
             continue;
+          }
         }
         break;
       }

packages/flutter/test/widgets/focus_scope_test.dart

@@ -1596,7 +1596,7 @@ void main() {
       bool? keyEventHandled;
       await tester.pumpWidget(
         Focus(
-          onKey: (_, __) => KeyEventResult.ignored, // This one does nothing.
+          onKey: (_, __) => true, // This one does nothing.
           focusNode: focusNode,
           child: Container(key: key1),
         ),
@@ -1611,7 +1611,7 @@ void main() {
         Focus(
           onKey: (FocusNode node, RawKeyEvent event) { // The updated handler handles the key.
             keyEventHandled = true;
-            return KeyEventResult.handled;
+            return true;
           },
           focusNode: focusNode,
           child: Container(key: key1),