test: Prevent mutation of shared example users

This ensures we don't re-introduce the sort of state leak that
caused #1712.  In three recent commits we fixed every existing
example of this sort of state leak; only three test files had them.

I also looked through all the other top-level fields in this file
`example_data.dart`.  Most are immutable atoms like ints and strings.
There are a handful of remaining points of mutability which could in
principle allow this sort of state leak; so I'll tighten those up too
in the next couple of commits.

Author: gnprice

test/example_data.dart

@@ -17,6 +17,7 @@ import 'package:zulip/model/narrow.dart';
 import 'package:zulip/model/settings.dart';
 import 'package:zulip/model/store.dart';
 
+import 'model/binding.dart';
 import 'model/test_store.dart';
 import 'stdlib_checks.dart';
 
@@ -289,24 +290,77 @@ Account account({
   );
 }
 
-final User selfUser = user(fullName: 'Self User');
+/// A [User] which throws on attempting to mutate any of its fields.
+///
+/// We use this to prevent any tests from leaking state through having a
+/// [PerAccountStore] (which will be discarded when [TestZulipBinding.reset]
+/// is called at the end of the test case) mutate a [User] in its [UserStore]
+/// which happens to a value in this file like [selfUser] (which will not be
+/// discarded by [TestZulipBinding.reset]).  That was the cause of issue #1712.
+class _ImmutableUser extends User {
+  _ImmutableUser.copyUser(User user) : super(
+    // When adding a field here, be sure to add the corresponding setter below.
+    userId: user.userId,
+    deliveryEmail: user.deliveryEmail,
+    email: user.email,
+    fullName: user.fullName,
+    dateJoined: user.dateJoined,
+    isActive: user.isActive,
+    isBillingAdmin: user.isBillingAdmin,
+    isBot: user.isBot,
+    botType: user.botType,
+    botOwnerId: user.botOwnerId,
+    role: user.role,
+    timezone: user.timezone,
+    avatarUrl: user.avatarUrl,
+    avatarVersion: user.avatarVersion,
+    profileData: user.profileData == null ? null : Map.unmodifiable(user.profileData!),
+    isSystemBot: user.isSystemBot,
+    // When adding a field here, be sure to add the corresponding setter below.
+  );
+
+  static final Error _error = UnsupportedError(
+    'Cannot mutate immutable User.\n'
+    'When a test needs to have the store handle an event which will\n'
+    'modify a user, use `eg.user()` to make a fresh User object\n'
+    'instead of using a shared User object like `eg.selfUser`.');
+
+  // userId already immutable
+  @override set deliveryEmail(_) => throw _error;
+  @override set email(_) => throw _error;
+  @override set fullName(_) => throw _error;
+  // dateJoined already immutable
+  @override set isActive(_) => throw _error;
+  @override set isBillingAdmin(_) => throw _error;
+  // isBot already immutable
+  // botType already immutable
+  @override set botOwnerId(_) => throw _error;
+  @override set role(_) => throw _error;
+  @override set timezone(_) => throw _error;
+  @override set avatarUrl(_) => throw _error;
+  @override set avatarVersion(_) => throw _error;
+  @override set profileData(_) => throw _error;
+  // isSystemBot already immutable
+}
+
+final User selfUser = _ImmutableUser.copyUser(user(fullName: 'Self User'));
+final User otherUser = _ImmutableUser.copyUser(user(fullName: 'Other User'));
+final User thirdUser = _ImmutableUser.copyUser(user(fullName: 'Third User'));
+final User fourthUser  = _ImmutableUser.copyUser(user(fullName: 'Fourth User'));
+
+// There's no need for an [Account] analogue of [_ImmutableUser],
+// because [Account] (which is generated by Drift) is already immutable.
 final Account selfAccount = account(
   id: 1001,
   user: selfUser,
   apiKey: 'dQcEJWTq3LczosDkJnRTwf31zniGvMrO', // A Zulip API key is 32 digits of base64.
 );
-
-final User otherUser = user(fullName: 'Other User');
 final Account otherAccount = account(
   id: 1002,
   user: otherUser,
   apiKey: '6dxT4b73BYpCTU+i4BB9LAKC5h/CufqY', // A Zulip API key is 32 digits of base64.
 );
 
-final User thirdUser = user(fullName: 'Third User');
-
-final User fourthUser  = user(fullName: 'Fourth User');
-
 ////////////////////////////////////////////////////////////////
 // Data attached to the self-account on the realm
 //