[web] Make glassPaneElement and glassPaneShadow non-nullable

lib/web_ui/lib/src/engine/embedder.dart

@@ -117,12 +117,12 @@ class FlutterViewEmbedder {
   /// which captures semantics input events. The semantics DOM tree must be a
   /// child of the glass pane element so that events bubble up to the glass pane
   /// if they are not handled by semantics.
-  DomElement? get glassPaneElement => _glassPaneElement;
-  DomElement? _glassPaneElement;
+  DomElement get glassPaneElement => _glassPaneElement;
+  late DomElement _glassPaneElement;
 
   /// The [HostNode] of the [glassPaneElement], which contains the whole Flutter app.
-  HostNode? get glassPaneShadow => _glassPaneShadow;
-  HostNode? _glassPaneShadow;
+  HostNode get glassPaneShadow => _glassPaneShadow;
+  late HostNode _glassPaneShadow;
 
   static const String defaultFontStyle = 'normal';
   static const String defaultFontWeight = 'normal';
@@ -149,9 +149,7 @@ class FlutterViewEmbedder {
     );
 
     // Create and inject the [_glassPaneElement].
-    final DomElement glassPaneElement =
-        domDocument.createElement(glassPaneTagName);
-    _glassPaneElement = glassPaneElement;
+    _glassPaneElement = domDocument.createElement(glassPaneTagName);
 
     // This must be attached to the DOM now, so the engine can create a host
     // node (ShadowDOM or a fallback) next.
@@ -340,8 +338,8 @@ class FlutterViewEmbedder {
         _embeddingStrategy.attachResourcesHost(resourcesHost,
             nextTo: glassPaneElement);
       } else {
-        glassPaneShadow!.node
-            .insertBefore(resourcesHost, glassPaneShadow!.node.firstChild);
+        glassPaneShadow.node
+            .insertBefore(resourcesHost, glassPaneShadow.node.firstChild);
       }
       _resourcesHost = resourcesHost;
     }

lib/web_ui/lib/src/engine/mouse_cursor.dart

@@ -67,7 +67,7 @@ class MouseCursor {
 
   void activateSystemCursor(String? kind) {
     setElementStyle(
-      flutterViewEmbedder.glassPaneElement!,
+      flutterViewEmbedder.glassPaneElement,
       'cursor',
       _mapKindToCssValue(kind),
     );

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -590,7 +590,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         _platformViewMessageHandler ??= PlatformViewMessageHandler(
           contentManager: platformViewManager,
           contentHandler: (DomElement content) {
-            flutterViewEmbedder.glassPaneElement!.append(content);
+            flutterViewEmbedder.glassPaneElement.append(content);
           },
         );
         _platformViewMessageHandler!.handlePlatformViewCall(data, callback!);

lib/web_ui/lib/src/engine/platform_views/content_manager.dart

@@ -161,7 +161,7 @@ class PlatformViewManager {
     final DomElement slot = domDocument.createElement('slot')
       ..style.display = 'none'
       ..setAttribute('name', tombstoneName);
-    flutterViewEmbedder.glassPaneShadow!.append(slot);
+    flutterViewEmbedder.glassPaneShadow.append(slot);
     // Link the element to the new slot
     element.setAttribute('slot', tombstoneName);
     // Delete both the element, and the new slot

lib/web_ui/lib/src/engine/semantics/text_field.dart

@@ -424,14 +424,14 @@ class TextField extends RoleManager {
         ..height = '${semanticsObject.rect!.height}px';
 
       if (semanticsObject.hasFocus) {
-        if (flutterViewEmbedder.glassPaneShadow!.activeElement !=
+        if (flutterViewEmbedder.glassPaneShadow.activeElement !=
             activeEditableElement) {
           semanticsObject.owner.addOneTimePostUpdateCallback(() {
             activeEditableElement.focus();
           });
         }
         SemanticsTextEditingStrategy.instance.activate(this);
-      } else if (flutterViewEmbedder.glassPaneShadow!.activeElement ==
+      } else if (flutterViewEmbedder.glassPaneShadow.activeElement ==
           activeEditableElement) {
         if (!isIosSafari) {
           SemanticsTextEditingStrategy.instance.deactivate(this);

lib/web_ui/lib/src/engine/text/measurement.dart

@@ -29,7 +29,7 @@ class RulerHost {
       ..height = '0';
 
     if (root == null) {
-      flutterViewEmbedder.glassPaneShadow!.node.appendChild(_rulerHost);
+      flutterViewEmbedder.glassPaneShadow.node.appendChild(_rulerHost);
     } else {
       root.appendChild(_rulerHost);
     }

lib/web_ui/lib/src/engine/text_editing/text_editing.dart

@@ -51,7 +51,7 @@ void _emptyCallback(dynamic _) {}
 
 /// The default [HostNode] that hosts all DOM required for text editing when a11y is not enabled.
 @visibleForTesting
-HostNode get defaultTextEditingRoot => flutterViewEmbedder.glassPaneShadow!;
+HostNode get defaultTextEditingRoot => flutterViewEmbedder.glassPaneShadow;
 
 /// These style attributes are constant throughout the life time of an input
 /// element.

lib/web_ui/test/canvaskit/embedded_views_test.dart

@@ -41,7 +41,7 @@ void testMain() {
       // The platform view is now split in two parts. The contents live
       // as a child of the glassPane, and the slot lives in the glassPane
       // shadow root. The slot is the one that has pointer events auto.
-      final DomElement contents = flutterViewEmbedder.glassPaneElement!
+      final DomElement contents = flutterViewEmbedder.glassPaneElement
           .querySelector('#view-0')!;
       final DomElement slot = flutterViewEmbedder.sceneElement!
           .querySelector('slot')!;
@@ -598,7 +598,7 @@ void testMain() {
       ]);
 
       expect(
-        flutterViewEmbedder.glassPaneElement!
+        flutterViewEmbedder.glassPaneElement
             .querySelector('flt-platform-view'),
         isNotNull,
       );
@@ -614,7 +614,7 @@ void testMain() {
       ]);
 
       expect(
-        flutterViewEmbedder.glassPaneElement!
+        flutterViewEmbedder.glassPaneElement
             .querySelector('flt-platform-view'),
         isNull,
       );
@@ -685,7 +685,7 @@ void testMain() {
       ]);
 
       expect(
-        flutterViewEmbedder.glassPaneElement!
+        flutterViewEmbedder.glassPaneElement
             .querySelector('flt-platform-view'),
         isNotNull,
       );
@@ -703,7 +703,7 @@ void testMain() {
       ]);
 
       expect(
-          flutterViewEmbedder.glassPaneElement!
+          flutterViewEmbedder.glassPaneElement
               .querySelectorAll('flt-platform-view'),
           hasLength(2));
 
@@ -719,7 +719,7 @@ void testMain() {
       // The actual contents of the platform view are kept in the dom, until
       // it's actually disposed of!
       expect(
-          flutterViewEmbedder.glassPaneElement!
+          flutterViewEmbedder.glassPaneElement
               .querySelectorAll('flt-platform-view'),
           hasLength(2));
     });

lib/web_ui/test/embedder_test.dart

@@ -46,14 +46,14 @@ void testMain() {
     final FlutterViewEmbedder embedder = FlutterViewEmbedder();
 
     expect(
-      embedder.glassPaneShadow?.querySelectorAll('flt-semantics-placeholder'),
+      embedder.glassPaneShadow.querySelectorAll('flt-semantics-placeholder'),
       isNotEmpty,
     );
   });
 
   test('renders a shadowRoot by default', () {
     final FlutterViewEmbedder embedder = FlutterViewEmbedder();
-    final HostNode hostNode = embedder.glassPaneShadow!;
+    final HostNode hostNode = embedder.glassPaneShadow;
     expect(domInstanceOfString(hostNode.node, 'ShadowRoot'), isTrue);
   });
 
@@ -64,7 +64,7 @@ void testMain() {
     attachShadow = null; // Break ShadowDOM
 
     final FlutterViewEmbedder embedder = FlutterViewEmbedder();
-    final HostNode hostNode = embedder.glassPaneShadow!;
+    final HostNode hostNode = embedder.glassPaneShadow;
     expect(domInstanceOfString(hostNode.node, 'Element'), isTrue);
     expect(
       (hostNode.node as DomElement).tagName,
@@ -92,7 +92,7 @@ void testMain() {
 
     regularTextField.focus();
     DomCSSStyleDeclaration? style = domWindow.getComputedStyle(
-        embedder.glassPaneShadow!.querySelector('input')!,
+        embedder.glassPaneShadow.querySelector('input')!,
         '::placeholder');
     expect(style, isNotNull);
     expect(style.opacity, isNot('0'));
@@ -104,7 +104,7 @@ void testMain() {
 
     textField.focus();
     style = domWindow.getComputedStyle(
-        embedder.glassPaneShadow!.querySelector('input.flt-text-editing')!,
+        embedder.glassPaneShadow.querySelector('input.flt-text-editing')!,
         '::placeholder');
     expect(style, isNotNull);
     expect(style.opacity, '0');

lib/web_ui/test/engine/pointer_binding_test.dart

@@ -42,7 +42,7 @@ void main() {
 
 void testMain() {
   ensureFlutterViewEmbedderInitialized();
-  final DomElement glassPane = flutterViewEmbedder.glassPaneElement!;
+  final DomElement glassPane = flutterViewEmbedder.glassPaneElement;
   late double dpi;
 
   setUp(() {

lib/web_ui/test/engine/semantics/semantics_test.dart

@@ -1646,7 +1646,7 @@ void _testTappable() {
     );
     tester.apply();
 
-    expect(flutterViewEmbedder.glassPaneShadow!.activeElement, tester.getSemanticsObject(0).element);
+    expect(flutterViewEmbedder.glassPaneShadow.activeElement, tester.getSemanticsObject(0).element);
     semantics().semanticsEnabled = false;
   });
 }
@@ -2007,7 +2007,7 @@ void _testPlatformView() {
     expect(child3Rect.bottom, 60);
 
     final DomElement platformViewElement =
-        flutterViewEmbedder.glassPaneElement!.querySelector('#view-0')!;
+        flutterViewEmbedder.glassPaneElement.querySelector('#view-0')!;
     final DomRect platformViewRect = platformViewElement.getBoundingClientRect();
     expect(platformViewRect.left, 0);
     expect(platformViewRect.top, 15);

lib/web_ui/test/engine/semantics/semantics_tester.dart

@@ -22,7 +22,7 @@ import '../../matchers.dart';
 /// so we don't have to hardcode domDocument across the test. (The host of a
 /// normal flutter app used to be domDocument, but now that the app is wrapped
 /// in a Shadow DOM, that's not the case anymore.)
-HostNode get appHostNode => flutterViewEmbedder.glassPaneShadow!;
+HostNode get appHostNode => flutterViewEmbedder.glassPaneShadow;
 
 /// CSS style applied to the root of the semantics tree.
 // TODO(yjbanov): this should be handled internally by [expectSemanticsTree].

lib/web_ui/test/html/bitmap_canvas_golden_test.dart

@@ -32,13 +32,13 @@ Future<void> testMain() async {
       testScene.style.transform = 'scale(0.3)';
     }
     testScene.append(canvas.rootElement);
-    flutterViewEmbedder.glassPaneShadow!.querySelector('flt-scene-host')!.append(testScene);
+    flutterViewEmbedder.glassPaneShadow.querySelector('flt-scene-host')!.append(testScene);
   }
 
   setUpStableTestFonts();
 
   tearDown(() {
-    flutterViewEmbedder.glassPaneShadow?.querySelector('flt-scene')?.remove();
+    flutterViewEmbedder.glassPaneShadow.querySelector('flt-scene')?.remove();
   });
 
   /// Draws several lines, some aligned precisely with the pixel grid, and some
@@ -260,7 +260,7 @@ Future<void> testMain() async {
     }
 
     sceneElement.querySelector('flt-clip')!.append(canvas.rootElement);
-    flutterViewEmbedder.glassPaneShadow!.querySelector('flt-scene-host')!.append(sceneElement);
+    flutterViewEmbedder.glassPaneShadow.querySelector('flt-scene-host')!.append(sceneElement);
 
     await matchGoldenFile(
       'bitmap_canvas_draws_text_on_top_of_canvas.png',