channel [nfc]: Expose debugTopicVisibility directly on store

This follows the pattern we generally use for other members of
ChannelStore and MessageStore, and lets us eliminate the getter
returning a ChannelStoreImpl.

This pattern -- exposing all the data as members directly on the
overall PerAccountStore, delegating to the underlying channel store
and message store as needed -- is convenient because it means that
code consuming the store doesn't have to worry about which sub-area
of the store we've organized a given piece of data into: it's all
just methods on the store.

Noticed this was here while reviewing a PR that included a new
similar deviation from this pattern:
  zulip#909 (comment)

Author: gnprice

lib/model/channel.dart

@@ -1,3 +1,5 @@
+import 'package:flutter/foundation.dart';
+
 import '../api/model/events.dart';
 import '../api/model/initial_snapshot.dart';
 import '../api/model/model.dart';
@@ -41,6 +43,16 @@ mixin ChannelStore {
   /// [isTopicVisibleInStream] and [isTopicVisible].
   UserTopicVisibilityPolicy topicVisibilityPolicy(int streamId, String topic);
 
+  /// The raw data structure underlying [topicVisibilityPolicy].
+  ///
+  /// This is sometimes convenient for checks in tests.
+  /// It differs from [topicVisibilityPolicy] in on the one hand omitting
+  /// all topics where the value would be [UserTopicVisibilityPolicy.none],
+  /// and on the other hand being a concrete, finite data structure that
+  /// can be compared using `deepEquals`.
+  @visibleForTesting
+  Map<int, Map<String, UserTopicVisibilityPolicy>> get debugTopicVisibility;
+
   /// Whether this topic should appear when already focusing on its stream.
   ///
   /// This is determined purely by the user's visibility policy for the topic.
@@ -191,6 +203,9 @@ class ChannelStoreImpl with ChannelStore {
   @override
   final Map<int, Subscription> subscriptions;
 
+  @override
+  Map<int, Map<String, UserTopicVisibilityPolicy>> get debugTopicVisibility => topicVisibility;
+
   final Map<int, Map<String, UserTopicVisibilityPolicy>> topicVisibility;
 
   @override

lib/model/store.dart

@@ -355,12 +355,12 @@ class PerAccountStore extends ChangeNotifier with ChannelStore, MessageStore {
   @override
   UserTopicVisibilityPolicy topicVisibilityPolicy(int streamId, String topic) =>
     _channels.topicVisibilityPolicy(streamId, topic);
+  @override
+  Map<int, Map<String, UserTopicVisibilityPolicy>> get debugTopicVisibility =>
+    _channels.debugTopicVisibility;
 
   final ChannelStoreImpl _channels;
 
-  @visibleForTesting
-  ChannelStoreImpl get debugChannelStore => _channels;
-
   ////////////////////////////////
   // Messages, and summaries of messages.
 

test/model/channel_test.dart

@@ -295,8 +295,8 @@ void main() {
       final expectedStore = eg.store(initialSnapshot: eg.initialSnapshot(
         userTopics: expected,
       ));
-      check(store.debugChannelStore.topicVisibility)
-        .deepEquals(expectedStore.debugChannelStore.topicVisibility);
+      check(store.debugTopicVisibility)
+        .deepEquals(expectedStore.debugTopicVisibility);
     }
 
     test('data structure', () {
@@ -308,7 +308,7 @@ void main() {
           eg.userTopicItem(stream2, 'topic 3', UserTopicVisibilityPolicy.unknown),
           eg.userTopicItem(stream2, 'topic 4', UserTopicVisibilityPolicy.followed),
         ]));
-      check(store.debugChannelStore.topicVisibility).deepEquals({
+      check(store.debugTopicVisibility).deepEquals({
         stream1.streamId: {
           'topic 1': UserTopicVisibilityPolicy.muted,
           'topic 2': UserTopicVisibilityPolicy.unmuted,