Enable copy to clipboard for web, as it's added to Flutter master channel (#253)

gallery/gallery/lib/pages/demo.dart

@@ -299,10 +299,6 @@ class _DemoPageState extends State<DemoPage> with TickerProviderStateMixin {
             maxHeight: maxSectionHeight,
             codeWidget: CodeDisplayPage(
               _currentConfig.code,
-              hasCopyButton:
-                  !kIsWeb, // TODO: Add support for copying code in Web.
-              // TODO: It is a known issue that Clipboard does not work on web.
-              // TODO: https://github.com/flutter/flutter/issues/40124
             ),
           ),
         );
@@ -701,10 +697,9 @@ class _DemoSectionCode extends StatelessWidget {
 }
 
 class CodeDisplayPage extends StatelessWidget {
-  const CodeDisplayPage(this.code, {this.hasCopyButton = true});
+  const CodeDisplayPage(this.code);
 
   final CodeDisplayer code;
-  final bool hasCopyButton;
 
   Widget build(BuildContext context) {
     final isDesktop = isDisplayDesktop(context);
@@ -723,7 +718,7 @@ class CodeDisplayPage extends StatelessWidget {
       );
     }
 
-    void _showSnackBarOnCopyFailure(Exception exception) {
+    void _showSnackBarOnCopyFailure(Object exception) {
       Scaffold.of(context).showSnackBar(
         SnackBar(
           content: Text(
@@ -737,7 +732,6 @@ class CodeDisplayPage extends StatelessWidget {
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
       children: [
-        if (hasCopyButton)
         Padding(
           padding: isDesktop
               ? EdgeInsets.only(bottom: 8)
@@ -750,6 +744,8 @@ class CodeDisplayPage extends StatelessWidget {
               borderRadius: BorderRadius.all(Radius.circular(4)),
             ),
             onPressed: () async {
+              // The future will not complete on web, thus no
+              // Snackbar will be shown, see https://github.com/flutter/flutter/issues/49349.
               await Clipboard.setData(ClipboardData(text: _plainTextCode))
                   .then(_showSnackBarOnCopySuccess)
                   .catchError(_showSnackBarOnCopyFailure);