store: Extract self-user up front; require to be present

We'll want to pass this to RealmStoreImpl soon, in order to interpret
permissions correctly.

In the series of commits leading up to here, we've already adapted all
the tests' data to satisfy this invariant.

Author: gnprice

lib/model/store.dart

@@ -481,8 +481,22 @@ class PerAccountStore extends PerAccountStoreBase with
       accountId: accountId,
       selfUserId: account.userId,
     );
-    final realm = RealmStoreImpl(core: core, initialSnapshot: initialSnapshot);
+
     final userMap = UserStoreImpl.userMapFromInitialSnapshot(initialSnapshot);
+    final selfUser = userMap[core.selfUserId];
+    if (selfUser == null) {
+      final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+      reportErrorToUserModally(
+        zulipLocalizations.errorCouldNotConnectTitle,
+        message: zulipLocalizations.errorMalformedResponseWithCause(200,
+          // skip-i18n: This would be an unlikely bug (in the server?).  We're
+          //   showing the user these details at all only because it would be a
+          //   very nasty bug (so, important to resolve ASAP) if it ever did happen.
+          'self-user missing from user list'));
+      throw Exception("bad initial snapshot: self-user missing from user list");
+    }
+
+    final realm = RealmStoreImpl(core: core, initialSnapshot: initialSnapshot);
     final users = UserStoreImpl(realm: realm, initialSnapshot: initialSnapshot,
       userMap: userMap);
     final channels = ChannelStoreImpl(users: users,