action_sheet [nfc]: Pull out base class for message action sheet buttons

We're about to add another button, for quote-and-reply zulip#116.

lib/widgets/action_sheet.dart

@@ -9,28 +9,59 @@ void showMessageActionSheet({required BuildContext context, required Message mes
     context: context,
     builder: (BuildContext innerContext) {
       return Column(children: [
-        MenuItemButton(
-          leadingIcon: Icon(Icons.adaptive.share),
-          onPressed: () async {
-            // Close the message action sheet; we're about to show the share
-            // sheet. (We could do this after the sharing Future settles, but
-            // on iOS I get impatient with how slowly our action sheet
-            // dismisses in that case.)
-            // TODO(#24): Fix iOS bug where this call causes the keyboard to
-            //   reopen (if it was open at the time of this
-            //   `showMessageActionSheet` call) and cover a large part of the
-            //   share sheet.
-            Navigator.of(innerContext).pop();
-
-            // TODO: to support iPads, we're asked to give a
-            //   `sharePositionOrigin` param, or risk crashing / hanging:
-            //     https://pub.dev/packages/share_plus#ipad
-            //   Perhaps a wart in the API; discussion:
-            //     https://github.com/zulip/zulip-flutter/pull/12#discussion_r1130146231
-            // TODO: Share raw Markdown, not HTML
-            await Share.shareWithResult(message.content);
-          },
-          child: const Text('Share')),
+        ShareButton(message: message),
       ]);
     });
 }
+
+abstract class MessageActionSheetMenuItemButton extends StatelessWidget {
+  const MessageActionSheetMenuItemButton({
+    super.key,
+    required this.message,
+  });
+
+  IconData get icon;
+  String get label;
+  void Function(BuildContext) get onPressed;
+
+  final Message message;
+
+  @override
+  Widget build(BuildContext context) {
+    return MenuItemButton(
+      leadingIcon: Icon(icon),
+      onPressed: () => onPressed(context),
+      child: Text(label));
+  }
+}
+
+class ShareButton extends MessageActionSheetMenuItemButton {
+  const ShareButton({
+    super.key,
+    required super.message,
+  });
+
+  @override get icon => Icons.adaptive.share;
+
+  @override get label => 'Share';
+
+  @override get onPressed => (BuildContext context) async {
+    // Close the message action sheet; we're about to show the share
+    // sheet. (We could do this after the sharing Future settles, but
+    // on iOS I get impatient with how slowly our action sheet
+    // dismisses in that case.)
+    // TODO(#24): Fix iOS bug where this call causes the keyboard to
+    //   reopen (if it was open at the time of this
+    //   `showMessageActionSheet` call) and cover a large part of the
+    //   share sheet.
+    Navigator.of(context).pop();
+
+    // TODO: to support iPads, we're asked to give a
+    //   `sharePositionOrigin` param, or risk crashing / hanging:
+    //     https://pub.dev/packages/share_plus#ipad
+    //   Perhaps a wart in the API; discussion:
+    //     https://github.com/zulip/zulip-flutter/pull/12#discussion_r1130146231
+    // TODO: Share raw Markdown, not HTML
+    await Share.shareWithResult(message.content);
+  };
+}