Convert some widgets tests to NNBD (#67782)

Migrating some more widget tests to NNBD.

packages/flutter/test/widgets/ensure_visible_test.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'package:flutter_test/flutter_test.dart';

packages/flutter/test/widgets/error_widget_builder_test.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 

packages/flutter/test/widgets/fade_transition_test.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -12,8 +10,8 @@ void main() {
   testWidgets('FadeTransition', (WidgetTester tester) async {
     final DebugPrintCallback oldPrint = debugPrint;
     final List<String> log = <String>[];
-    debugPrint = (String message, { int wrapWidth }) {
-      log.add(message);
+    debugPrint = (String? message, { int? wrapWidth }) {
+      log.add(message!);
     };
     debugPrintBuildScope = true;
     final AnimationController controller = AnimationController(

packages/flutter/test/widgets/fitted_box_test.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -595,12 +593,11 @@ void main() {
 
 List<Type> getLayers() {
   final List<Type> layers = <Type>[];
-  Layer layer = RendererBinding.instance.renderView.debugLayer;
-  while (layer is ContainerLayer) {
-    final ContainerLayer container = layer as ContainerLayer;
+  Layer? container = RendererBinding.instance!.renderView.debugLayer;
+  while (container is ContainerLayer) {
     layers.add(container.runtimeType);
     expect(container.firstChild, same(container.lastChild));
-    layer = container.firstChild;
+    container = container.firstChild;
   }
   return layers;
 }

packages/flutter/test/widgets/flex_test.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -63,18 +61,6 @@ void main() {
     expect(box.size.width, 100.0);
   });
 
-  testWidgets('Can pass null for flex', (WidgetTester tester) async {
-    await tester.pumpWidget(
-      Row(
-        textDirection: TextDirection.ltr,
-        children: const <Widget>[
-          Expanded(flex: null, child: Text('one', textDirection: TextDirection.ltr)),
-          Flexible(flex: null, child: Text('two', textDirection: TextDirection.ltr)),
-        ],
-      ),
-    );
-  });
-
   testWidgets("Doesn't overflow because of floating point accumulated error", (WidgetTester tester) async {
     // both of these cases have failed in the past due to floating point issues
     await tester.pumpWidget(

packages/flutter/test/widgets/flow_test.dart

@@ -2,14 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 class TestFlowDelegate extends FlowDelegate {
-  TestFlowDelegate({this.startOffset}) : super(repaint: startOffset);
+  TestFlowDelegate({required this.startOffset}) : super(repaint: startOffset);
 
   final Animation<double> startOffset;
 
@@ -23,7 +21,7 @@ class TestFlowDelegate extends FlowDelegate {
     double dy = startOffset.value;
     for (int i = 0; i < context.childCount; ++i) {
       context.paintChild(i, transform: Matrix4.translationValues(0.0, dy, 0.0));
-      dy += 0.75 * context.getChildSize(i).height;
+      dy += 0.75 * context.getChildSize(i)!.height;
     }
   }
 
@@ -149,13 +147,13 @@ void main() {
         ],
       ),
     );
-    ContainerLayer layer = RendererBinding.instance.renderView.debugLayer;
+    ContainerLayer? layer = RendererBinding.instance!.renderView.debugLayer;
     while (layer != null && layer is! OpacityLayer)
-      layer = layer.firstChild as ContainerLayer;
+      layer = layer.firstChild as ContainerLayer?;
     expect(layer, isA<OpacityLayer>());
-    final OpacityLayer opacityLayer = layer as OpacityLayer;
-    expect(opacityLayer.alpha, equals(opacity * 255));
-    expect(layer.firstChild, isA<TransformLayer>());
+    final OpacityLayer? opacityLayer = layer as OpacityLayer?;
+    expect(opacityLayer!.alpha, equals(opacity * 255));
+    expect(layer!.firstChild, isA<TransformLayer>());
   });
 
   testWidgets('Flow can set and update clipBehavior', (WidgetTester tester) async {

packages/flutter/test/widgets/focus_manager_test.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
@@ -18,7 +16,7 @@ void main() {
   final GlobalKey widgetKey = GlobalKey();
   Future<BuildContext> setupWidget(WidgetTester tester) async {
     await tester.pumpWidget(Container(key: widgetKey));
-    return widgetKey.currentContext;
+    return widgetKey.currentContext!;
   }
 
   group(FocusNode, () {
@@ -931,7 +929,7 @@ void main() {
     }, variant: TargetPlatformVariant.all());
     testWidgets('Mouse events change initial focus highlight mode on mobile.', (WidgetTester tester) async {
       expect(FocusManager.instance.highlightMode, equals(FocusHighlightMode.touch));
-      RendererBinding.instance.initMouseTracker(); // Clear out the mouse state.
+      RendererBinding.instance!.initMouseTracker(); // Clear out the mouse state.
       final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse, pointer: 0);
       addTearDown(gesture.removePointer);
       await gesture.moveTo(Offset.zero);
@@ -939,7 +937,7 @@ void main() {
     }, variant: TargetPlatformVariant.mobile());
     testWidgets('Mouse events change initial focus highlight mode on desktop.', (WidgetTester tester) async {
       expect(FocusManager.instance.highlightMode, equals(FocusHighlightMode.traditional));
-      RendererBinding.instance.initMouseTracker(); // Clear out the mouse state.
+      RendererBinding.instance!.initMouseTracker(); // Clear out the mouse state.
       final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse, pointer: 0);
       addTearDown(gesture.removePointer);
       await gesture.moveTo(Offset.zero);
@@ -952,7 +950,7 @@ void main() {
     testWidgets('Events change focus highlight mode.', (WidgetTester tester) async {
       await setupWidget(tester);
       int callCount = 0;
-      FocusHighlightMode lastMode;
+      FocusHighlightMode? lastMode;
       void handleModeChange(FocusHighlightMode mode) {
         lastMode = mode;
         callCount++;