Make TextInputClient a mixin (#104291)

TextInputClient and DeltaTextInputClient are now mixins, which helps with breaking changes and future deltas work

packages/flutter/lib/src/services/text_input.dart

@@ -1086,11 +1086,7 @@ mixin TextSelectionDelegate {
 ///  * [EditableText], a [TextInputClient] implementation.
 ///  * [DeltaTextInputClient], a [TextInputClient] extension that receives
 ///    granular information from the platform's text input.
-abstract class TextInputClient {
-  /// Abstract const constructor. This constructor enables subclasses to provide
-  /// const constructors so that they can be used in const expressions.
-  const TextInputClient();
-
+mixin TextInputClient {
   /// The current state of the [TextEditingValue] held by this client.
   TextEditingValue? get currentTextEditingValue;
 
@@ -1225,7 +1221,7 @@ class SelectionRect {
 ///  * [TextInputConfiguration], to opt-in to receive [TextEditingDelta]'s from
 ///    the platforms [TextInput] you must set [TextInputConfiguration.enableDeltaModel]
 ///    to true.
-abstract class DeltaTextInputClient extends TextInputClient {
+mixin DeltaTextInputClient implements TextInputClient {
   /// Requests that this client update its editing state by applying the deltas
   /// received from the engine.
   ///

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

@@ -1676,7 +1676,7 @@ class EditableText extends StatefulWidget {
 }
 
 /// State for a [EditableText].
-class EditableTextState extends State<EditableText> with AutomaticKeepAliveClientMixin<EditableText>, WidgetsBindingObserver, TickerProviderStateMixin<EditableText>, TextSelectionDelegate implements TextInputClient, AutofillClient {
+class EditableTextState extends State<EditableText> with AutomaticKeepAliveClientMixin<EditableText>, WidgetsBindingObserver, TickerProviderStateMixin<EditableText>, TextSelectionDelegate, TextInputClient implements AutofillClient {
   Timer? _cursorTimer;
   AnimationController get _cursorBlinkOpacityController {
     return _backingCursorBlinkOpacityController ??= AnimationController(

packages/flutter/test/services/text_input_test.dart

@@ -701,7 +701,7 @@ void main() {
   });
 }
 
-class FakeTextInputClient implements TextInputClient {
+class FakeTextInputClient with TextInputClient {
   FakeTextInputClient(this.currentTextEditingValue);
 
   String latestMethodCall = '';