msglist [nfc]: Cut default for MessageListView.anchor

There's no value that's a natural default for this at a model level:
different UI scenarios will use different values.  So require callers
to be explicit.

Author: gnprice

lib/model/message_list.dart

@@ -450,7 +450,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   factory MessageListView.init({
     required PerAccountStore store,
     required Narrow narrow,
-    Anchor anchor = AnchorCode.newest, // TODO(#82): make required, for explicitness
+    required Anchor anchor,
   }) {
     return MessageListView._(store: store, narrow: narrow, anchor: anchor)
       .._register();

test/model/message_list_test.dart

@@ -1381,12 +1381,14 @@ void main() {
 
       int notifiedCount1 = 0;
       final model1 = MessageListView.init(store: store,
-          narrow: ChannelNarrow(stream.streamId))
+          narrow: ChannelNarrow(stream.streamId),
+          anchor: AnchorCode.newest)
         ..addListener(() => notifiedCount1++);
 
       int notifiedCount2 = 0;
       final model2 = MessageListView.init(store: store,
-          narrow: eg.topicNarrow(stream.streamId, 'hello'))
+          narrow: eg.topicNarrow(stream.streamId, 'hello'),
+          anchor: AnchorCode.newest)
         ..addListener(() => notifiedCount2++);
 
       for (final m in [model1, model2]) {
@@ -1426,7 +1428,8 @@ void main() {
       await store.handleEvent(mkEvent(message));
 
       // init msglist *after* event was handled
-      model = MessageListView.init(store: store, narrow: const CombinedFeedNarrow());
+      model = MessageListView.init(store: store,
+        narrow: const CombinedFeedNarrow(), anchor: AnchorCode.newest);
       checkInvariants(model);
 
       connection.prepare(json:

test/model/message_test.dart

@@ -51,7 +51,6 @@ void main() {
 
   /// Initialize [store] and the rest of the test state.
   Future<void> prepare({
-    Narrow narrow = const CombinedFeedNarrow(),
     ZulipStream? stream,
     int? zulipFeatureLevel,
   }) async {
@@ -64,7 +63,9 @@ void main() {
     await store.addSubscription(subscription);
     connection = store.connection as FakeApiConnection;
     notifiedCount = 0;
-    messageList = MessageListView.init(store: store, narrow: narrow)
+    messageList = MessageListView.init(store: store,
+        narrow: const CombinedFeedNarrow(),
+        anchor: AnchorCode.newest)
       ..addListener(() {
         notifiedCount++;
       });