Remove assert that prevents WidgetSpans from being used in SelectableText (#92295)

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

@@ -585,9 +585,11 @@ class _SelectableTextState extends State<SelectableText> with AutomaticKeepAlive
   @override
   Widget build(BuildContext context) {
     super.build(context); // See AutomaticKeepAliveClientMixin.
-    assert(() {
-      return _controller._textSpan.visitChildren((InlineSpan span) => span.runtimeType == TextSpan);
-    }(), 'SelectableText only supports TextSpan; Other type of InlineSpan is not allowed');
+    // TODO(garyq): Assert to block WidgetSpans from being used here are removed,
+    // but we still do not yet have nice handling of things like carets, clipboard,
+    // and other features. We should add proper support. Currently, caret handling
+    // is blocked on SkParagraph switch and https://github.com/flutter/engine/pull/27010
+    // should be landed in SkParagraph after the switch is complete.
     assert(debugCheckHasMediaQuery(context));
     assert(debugCheckHasDirectionality(context));
     assert(

packages/flutter/test/widgets/selectable_text_test.dart

@@ -319,7 +319,7 @@ void main() {
     expect(selectableText.enableInteractiveSelection, true);
   });
 
-  testWidgets('Rich selectable text only support TextSpan', (WidgetTester tester) async {
+  testWidgets('Rich selectable text supports WidgetSpan', (WidgetTester tester) async {
     await tester.pumpWidget(
       const MediaQuery(
         data: MediaQueryData(),
@@ -357,7 +357,7 @@ void main() {
         ),
       ),
     );
-    expect(tester.takeException(), isAssertionError);
+    expect(tester.takeException(), isNull);
   });
 
   testWidgets('no text keyboard when widget is focused', (WidgetTester tester) async {