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

@@ -420,7 +420,7 @@ 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

@@ -1368,12 +1368,14 @@ 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]) {
@@ -1413,7 +1415,8 @@ 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

@@ -42,15 +42,17 @@ void main() {
   void checkNotifiedOnce() => checkNotified(count: 1);
 
   /// Initialize [store] and the rest of the test state.
-  Future<void> prepare({Narrow narrow = const CombinedFeedNarrow()}) async {
+  Future<void> prepare() async {
     final stream = eg.stream(streamId: eg.defaultStreamMessageStreamId);
     subscription = eg.subscription(stream);
     store = eg.store();
     await store.addStream(stream);
     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++;
       });