Remove TextEditingController private member access (#149042)

Fixes flutter/flutter#148692

I intend to CP this.

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

@@ -314,7 +314,7 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// If the new selection is outside the composing range, the composing range is
   /// cleared.
   set selection(TextSelection newSelection) {
-    if (!_isSelectionWithinTextBounds(newSelection)) {
+    if (text.length < newSelection.end || text.length < newSelection.start) {
       throw FlutterError('invalid text selection: $newSelection');
     }
     final TextRange newComposing = _isSelectionWithinComposingRange(newSelection) ? value.composing : TextRange.empty;
@@ -348,11 +348,6 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
     value = value.copyWith(composing: TextRange.empty);
   }
 
-  /// Check that the [selection] is inside of the bounds of [text].
-  bool _isSelectionWithinTextBounds(TextSelection selection) {
-    return selection.start <= text.length && selection.end <= text.length;
-  }
-
   /// Check that the [selection] is inside of the composing range.
   bool _isSelectionWithinComposingRange(TextSelection selection) {
     return selection.start >= value.composing.start && selection.end <= value.composing.end;
@@ -3953,7 +3948,8 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     // We return early if the selection is not valid. This can happen when the
     // text of [EditableText] is updated at the same time as the selection is
     // changed by a gesture event.
-    if (!widget.controller._isSelectionWithinTextBounds(selection)) {
+    final String text = widget.controller.value.text;
+    if (text.length < selection.end || text.length < selection.start) {
       return;
     }
 

packages/flutter/test/widgets/editable_text_test.dart

@@ -17654,6 +17654,27 @@ void main() {
 
     expect(tester.takeException(), isNull);
   });
+
+  testWidgets('Can implement TextEditingController', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MediaQuery(
+        data: const MediaQueryData(),
+        child: Directionality(
+          textDirection: TextDirection.ltr,
+          child: EditableText(
+            autofocus: true,
+            backgroundCursorColor: Colors.grey,
+            controller: _TextEditingControllerImpl(),
+            focusNode: focusNode,
+            style: textStyle,
+            cursorColor: cursorColor,
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
 }
 
 class UnsettableController extends TextEditingController {
@@ -18016,6 +18037,36 @@ class _AccentColorTextEditingController extends TextEditingController {
   }
 }
 
+class _TextEditingControllerImpl extends ChangeNotifier implements TextEditingController {
+  final TextEditingController _innerController = TextEditingController();
+
+  @override
+  void clear() => _innerController.clear();
+
+  @override
+  void clearComposing() => _innerController.clearComposing();
+
+  @override
+  TextSelection get selection => _innerController.selection;
+  @override
+  set selection(TextSelection newSelection) => _innerController.selection = newSelection;
+
+  @override
+  String get text => _innerController.text;
+  @override
+  set text(String newText) => _innerController.text = newText;
+
+  @override
+  TextSpan buildTextSpan({required BuildContext context, TextStyle? style, required bool withComposing}) {
+    return _innerController.buildTextSpan(context: context, style: style, withComposing: withComposing);
+  }
+
+  @override
+  TextEditingValue get value => _innerController.value;
+  @override
+  set value(TextEditingValue newValue) => _innerController.value = newValue;
+}
+
 class _TestScrollController extends ScrollController {
   bool get attached => hasListeners;
 }