message_list: Add search button to combined feed

Fixes: #1854

Author: Komal Singh

lib/widgets/message_list.dart

@@ -416,6 +416,12 @@ abstract class _MessageListAppBar {
     List<Widget> actions = [];
     switch (narrow) {
       case CombinedFeedNarrow():
+        actions.add(IconButton(
+          icon: const Icon(ZulipIcons.search),
+          tooltip: zulipLocalizations.searchMessagesPageTitle,
+          onPressed: () => Navigator.push(context,
+            MessageListPage.buildRoute(context: context,
+              narrow: KeywordSearchNarrow('')))));
       case MentionsNarrow():
       case StarredMessagesNarrow():
       case KeywordSearchNarrow():

test/widgets/message_list_test.dart

@@ -363,6 +363,35 @@ void main() {
 
       check(find.text('DMs with Muted user, User 2, Muted user')).findsOne();
     });
+
+    testWidgets('search button on combined feed navigates to search page', (tester) async {
+      final pushedRoutes = <Route<dynamic>>[];
+      final testNavObserver = TestNavigatorObserver()
+        ..onPushed = (route, prevRoute) => pushedRoutes.add(route);
+
+      await setupMessageListPage(tester,
+        narrow: const CombinedFeedNarrow(),
+        messages: [],
+        navObservers: [testNavObserver]);
+
+      final searchButtonFinder = find.descendant(
+        of: find.byType(ZulipAppBar),
+        matching: find.byIcon(ZulipIcons.search));
+      check(searchButtonFinder).findsOne();
+
+      pushedRoutes.clear();
+
+      connection.prepare(json: eg.newestGetMessagesResult(
+        foundOldest: true, messages: []).toJson());
+
+      await tester.tap(searchButtonFinder);
+      await tester.pump();
+
+      check(pushedRoutes).single.isA<WidgetRoute>().page
+        .isA<MessageListPage>()
+        .initNarrow.equals(KeywordSearchNarrow(''));
+      await tester.pump(Duration.zero);
+    });
   });
 
   group('no-messages placeholder', () {