msglist [nfc]: Pull out a _buildItem method

This will help keep down complexity (and depth of indentation)
as we make the list view more complex.  In particular it will
avoid duplication when we start having two back-to-back slivers
that both contain message-list items.

Author: gnprice

lib/widgets/message_list.dart

@@ -329,36 +329,40 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
         if (i == 1) return MarkAsReadWidget(narrow: widget.narrow);
 
         final data = model!.items[length - 1 - (i - 2)];
-        switch (data) {
-          case MessageListHistoryStartItem():
-            return const Center(
-              child: Padding(
-                padding: EdgeInsets.symmetric(vertical: 16.0),
-                child: Text("No earlier messages."))); // TODO use an icon
-          case MessageListLoadingItem():
-            return const Center(
-              child: Padding(
-                padding: EdgeInsets.symmetric(vertical: 16.0),
-                child: CircularProgressIndicator())); // TODO perhaps a different indicator
-          case MessageListRecipientHeaderItem():
-            final header = RecipientHeader(message: data.message, narrow: widget.narrow);
-            return StickyHeaderItem(allowOverflow: true,
-              header: header, child: header);
-          case MessageListDateSeparatorItem():
-            final header = RecipientHeader(message: data.message, narrow: widget.narrow);
-            return StickyHeaderItem(allowOverflow: true,
-              header: header,
-              child: DateSeparator(message: data.message));
-          case MessageListMessageItem():
-            final header = RecipientHeader(message: data.message, narrow: widget.narrow);
-            return MessageItem(
-              key: ValueKey(data.message.id),
-              header: header,
-              trailingWhitespace: i == 1 ? 8 : 11,
-              item: data);
-        }
+        return _buildItem(data, i);
       });
   }
+
+  Widget _buildItem(MessageListItem data, int i) {
+    switch (data) {
+      case MessageListHistoryStartItem():
+        return const Center(
+          child: Padding(
+            padding: EdgeInsets.symmetric(vertical: 16.0),
+            child: Text("No earlier messages."))); // TODO use an icon
+      case MessageListLoadingItem():
+        return const Center(
+          child: Padding(
+            padding: EdgeInsets.symmetric(vertical: 16.0),
+            child: CircularProgressIndicator())); // TODO perhaps a different indicator
+      case MessageListRecipientHeaderItem():
+        final header = RecipientHeader(message: data.message, narrow: widget.narrow);
+        return StickyHeaderItem(allowOverflow: true,
+          header: header, child: header);
+      case MessageListDateSeparatorItem():
+        final header = RecipientHeader(message: data.message, narrow: widget.narrow);
+        return StickyHeaderItem(allowOverflow: true,
+          header: header,
+          child: DateSeparator(message: data.message));
+      case MessageListMessageItem():
+        final header = RecipientHeader(message: data.message, narrow: widget.narrow);
+        return MessageItem(
+          key: ValueKey(data.message.id),
+          header: header,
+          trailingWhitespace: i == 1 ? 8 : 11,
+          item: data);
+    }
+  }
 }
 
 class ScrollToBottomButton extends StatelessWidget {