[web] Don't crash on const HtmlElementView() (flutter#128965)

Previously, when the code contained `const HtmlElementView()` it would break even if it's guarded by `if (kIsWeb)`.

This PR makes it such that `const HtmlElementView()` is allowed but it still throws if it gets inserted into the widget tree by mistake on non-web platforms.

One improvement we can make in the future is to have a conditional import:
- `_html_element_view_web.dart` that contains the real `HtmlElementView` that can only be instantiated on web.
- `_html_element_view_io.dart` that contains a stub with an unimplemented `build()` method.

Fixes flutter#43532

Author: mdebbar

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

@@ -346,7 +346,7 @@ class HtmlElementView extends StatelessWidget {
     required this.viewType,
     this.onPlatformViewCreated,
     this.creationParams,
-  }) : assert(kIsWeb, 'HtmlElementView is only available on Flutter Web.');
+  });
 
   /// The unique identifier for the HTML view type to be embedded by this widget.
   ///
@@ -363,6 +363,7 @@ class HtmlElementView extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    assert(kIsWeb, 'HtmlElementView is only available on Flutter Web.');
     return PlatformViewLink(
       viewType: viewType,
       onCreatePlatformView: _createHtmlElementView,

packages/flutter/test/widgets/platform_view_test.dart

@@ -3168,4 +3168,28 @@ void main() {
     expect(controller.dispatchedPointerEvents, hasLength(1));
     expect(controller.dispatchedPointerEvents[0], isA<PointerHoverEvent>());
   });
+
+  testWidgets('HtmlElementView can be instantiated', (WidgetTester tester) async {
+    late final Widget htmlElementView;
+    expect(() {
+      htmlElementView = const HtmlElementView(viewType: 'webview');
+    }, returnsNormally);
+
+    await tester.pumpWidget(
+      Center(
+        child: SizedBox(
+          width: 100,
+          height: 100,
+          child: htmlElementView,
+        ),
+      )
+    );
+    await tester.pumpAndSettle();
+
+    // This file runs on non-web platforms, so we expect `HtmlElementView` to
+    // fail.
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+    expect(exception.toString(), contains('HtmlElementView is only available on Flutter Web'));
+  });
 }