Update test Dart code to pass the latest Dart analyzer rules (#20986)

testing/dart/compositing_test.dart

@@ -326,7 +326,7 @@ void main() {
       );
     });
     testNoSharing((SceneBuilder builder, EngineLayer oldLayer) {
-      return builder.pushPhysicalShape(path: Path(), color: const Color.fromARGB(0, 0, 0, 0), oldLayer: oldLayer as PhysicalShapeEngineLayer);
+      return builder.pushPhysicalShape(path: Path(), color: const Color.fromARGB(0, 0, 0, 0), oldLayer: oldLayer as PhysicalShapeEngineLayer, elevation: 0.0);
     });
     testNoSharing((SceneBuilder builder, EngineLayer oldLayer) {
       return builder.pushColorFilter(

testing/scenario_app/lib/src/locale_initialization.dart

@@ -60,9 +60,16 @@ class LocaleInitialization extends Scenario {
         currentValueLength: 0,
         scrollChildren: 0,
         scrollIndex: 0,
+        scrollPosition: 0.0,
+        scrollExtentMax: 0.0,
+        scrollExtentMin: 0.0,
         transform: Matrix4.identity().storage,
         elevation: 0.0,
         thickness: 0.0,
+        hint: '',
+        value: '',
+        increasedValue: '',
+        decreasedValue: '',
         childrenInTraversalOrder: Int32List(0),
         childrenInHitTestOrder: Int32List(0),
         additionalActions: Int32List(0),
@@ -101,9 +108,16 @@ class LocaleInitialization extends Scenario {
         currentValueLength: 0,
         scrollChildren: 0,
         scrollIndex: 0,
+        scrollPosition: 0.0,
+        scrollExtentMax: 0.0,
+        scrollExtentMin: 0.0,
         transform: Matrix4.identity().storage,
         elevation: 0.0,
         thickness: 0.0,
+        hint: '',
+        value: '',
+        increasedValue: '',
+        decreasedValue: '',
         childrenInTraversalOrder: Int32List(0),
         childrenInHitTestOrder: Int32List(0),
         additionalActions: Int32List(0),

testing/scenario_app/lib/src/platform_echo_mixin.dart

@@ -8,6 +8,7 @@ import 'dart:ui';
 
 import 'scenario.dart';
 
+/// Echo platform messages back to the sender.
 mixin PlatformEchoMixin on Scenario {
   @override
   void onPlatformMessage(

testing/scenario_app/lib/src/send_text_focus_semantics.dart

@@ -55,9 +55,16 @@ class SendTextFocusScemantics extends Scenario {
         currentValueLength: 0,
         scrollChildren: 0,
         scrollIndex: 0,
+        scrollPosition: 0.0,
+        scrollExtentMax: 0.0,
+        scrollExtentMin: 0.0,
         transform: Matrix4.identity().storage,
         elevation: 0.0,
         thickness: 0.0,
+        hint: '',
+        value: '',
+        increasedValue: '',
+        decreasedValue: '',
         childrenInTraversalOrder: Int32List(0),
         childrenInHitTestOrder: Int32List(0),
         additionalActions: Int32List(0),
@@ -105,9 +112,16 @@ class SendTextFocusScemantics extends Scenario {
         currentValueLength: 0,
         scrollChildren: 0,
         scrollIndex: 0,
+        scrollPosition: 0.0,
+        scrollExtentMax: 0.0,
+        scrollExtentMin: 0.0,
         transform: Matrix4.identity().storage,
         elevation: 0.0,
         thickness: 0.0,
+        hint: '',
+        value: '',
+        increasedValue: '',
+        decreasedValue: '',
         childrenInTraversalOrder: Int32List(0),
         childrenInHitTestOrder: Int32List(0),
         additionalActions: Int32List(0),