user [nfc]: Expose UserStoreImpl.userMapFromInitialSnapshot

We'll need to pass RealmStoreImpl some details of the self-user, in
order to correctly interpret (for #814) the permissions that live
on RealmStore.

But because UserStore already depends on RealmStore, we don't want
to introduce a dependency in RealmStoreImpl on UserStore as a whole.
So separate out the step that processes the user lists, and do that
step in advance before constructing RealmStoreImpl.

Author: gnprice

lib/model/store.dart

@@ -482,7 +482,9 @@ class PerAccountStore extends PerAccountStoreBase with
       selfUserId: account.userId,
     );
     final realm = RealmStoreImpl(core: core, initialSnapshot: initialSnapshot);
-    final users = UserStoreImpl(realm: realm, initialSnapshot: initialSnapshot);
+    final userMap = UserStoreImpl.userMapFromInitialSnapshot(initialSnapshot);
+    final users = UserStoreImpl(realm: realm, initialSnapshot: initialSnapshot,
+      userMap: userMap);
     final channels = ChannelStoreImpl(users: users,
       initialSnapshot: initialSnapshot);
     return PerAccountStore._(

lib/model/user.dart

@@ -196,18 +196,27 @@ abstract class HasUserStore extends HasRealmStore with UserStore, ProxyUserStore
 /// itself.  Other code accesses this functionality through [PerAccountStore],
 /// or through the mixin [UserStore] which describes its interface.
 class UserStoreImpl extends HasRealmStore with UserStore {
+  /// Construct an implementation of [UserStore] that does the work itself.
+  ///
+  /// The `userMap` parameter should be the result of
+  /// [UserStoreImpl.userMapFromInitialSnapshot] applied to `initialSnapshot`.
   UserStoreImpl({
     required super.realm,
     required InitialSnapshot initialSnapshot,
-  }) : _users = Map.fromEntries(
-         initialSnapshot.realmUsers
-         .followedBy(initialSnapshot.realmNonActiveUsers)
-         .followedBy(initialSnapshot.crossRealmBots)
-         .map((user) => MapEntry(user.userId, user))),
+    required Map<int, User> userMap,
+  }) : _users = userMap,
        _mutedUsers = Set.from(initialSnapshot.mutedUsers.map((item) => item.id)),
        _userStatuses = initialSnapshot.userStatuses.map((userId, change) =>
          MapEntry(userId, change.apply(UserStatus.zero)));
 
+  static Map<int, User> userMapFromInitialSnapshot(InitialSnapshot initialSnapshot) {
+    return Map.fromEntries(
+      initialSnapshot.realmUsers
+      .followedBy(initialSnapshot.realmNonActiveUsers)
+      .followedBy(initialSnapshot.crossRealmBots)
+      .map((user) => MapEntry(user.userId, user)));
+  }
+
   final Map<int, User> _users;
 
   @override