api test: Test sendMessage's checking of the realm

This is mostly for the sake of starting to exercise FakeApiConnection,
though this is functionality that it's probably best to have a test
for in any case.

Author: gnprice

test/api/route/messages_test.dart

@@ -0,0 +1,26 @@
+import 'dart:convert';
+
+import 'package:checks/checks.dart';
+import 'package:test/scaffolding.dart';
+import 'package:zulip/api/route/messages.dart';
+
+import '../fake_api.dart';
+import 'route_checks.dart';
+
+void main() {
+  test('sendMessage accepts fixture realm', () async {
+    final connection = FakeApiConnection(
+        realmUrl: 'https://chat.zulip.org/', email: '[email protected]');
+    connection.prepare(jsonEncode(SendMessageResult(id: 42).toJson()));
+    check(sendMessage(connection, content: 'hello', topic: 'world'))
+        .completes(it()..id.equals(42));
+  });
+
+  test('sendMessage rejects unexpected realm', () async {
+    final connection = FakeApiConnection(
+        realmUrl: 'https://chat.example/', email: '[email protected]');
+    connection.prepare(jsonEncode(SendMessageResult(id: 42).toJson()));
+    check(sendMessage(connection, content: 'hello', topic: 'world'))
+        .throws();
+  });
+}

test/api/route/route_checks.dart

@@ -0,0 +1,11 @@
+// ignore_for_file: non_constant_identifier_names
+
+import 'package:checks/checks.dart';
+import 'package:zulip/api/route/messages.dart';
+
+extension SendMessageResultChecks on Subject<SendMessageResult> {
+  Subject<int> get id => has((e) => e.id, 'id');
+  Subject<String?> get deliver_at => has((e) => e.deliver_at, 'deliver_at');
+}
+
+// TODO add similar extensions for other classes in api/route/*.dart