test: Write a generic jsonEquals check and deepToJson function

test/api/model/events_test.dart

@@ -3,6 +3,7 @@ import 'package:test/scaffolding.dart';
 import 'package:zulip/api/model/events.dart';
 
 import '../../example_data.dart' as eg;
+import '../../stdlib_checks.dart';
 import 'events_checks.dart';
 import 'model_checks.dart';
 

test/api/model/model_checks.dart

@@ -2,14 +2,6 @@ import 'package:checks/checks.dart';
 import 'package:zulip/api/model/model.dart';
 
 extension MessageChecks on Subject<Message> {
-  Subject<Map<String, dynamic>> get toJson => has((e) => e.toJson(), 'toJson');
-
-  void jsonEquals(Message expected) {
-    final expectedJson = expected.toJson();
-    expectedJson['reactions'] = it()..isA<List<Reaction>>().jsonEquals(expected.reactions);
-    toJson.deepEquals(expectedJson);
-  }
-
   Subject<String> get content => has((e) => e.content, 'content');
   Subject<bool> get isMeMessage => has((e) => e.isMeMessage, 'isMeMessage');
   Subject<int?> get lastEditTimestamp => has((e) => e.lastEditTimestamp, 'lastEditTimestamp');
@@ -23,21 +15,9 @@ extension ReactionsChecks on Subject<List<Reaction>> {
   void deepEquals(_) {
     throw UnimplementedError('Tried to call [Subject<List<Reaction>>.deepEquals]. Use jsonEquals instead.');
   }
-
-  void jsonEquals(List<Reaction> expected) {
-    // (cast, to bypass this extension's deepEquals implementation, which throws)
-    // ignore: unnecessary_cast
-    (this as Subject<List>).deepEquals(expected.map((r) => it()..isA<Reaction>().jsonEquals(r)));
-  }
 }
 
 extension ReactionChecks on Subject<Reaction> {
-  Subject<Map<String, dynamic>> get toJson => has((r) => r.toJson(), 'toJson');
-
-  void jsonEquals(Reaction expected) {
-    toJson.deepEquals(expected.toJson());
-  }
-
   Subject<String> get emojiName => has((r) => r.emojiName, 'emojiName');
   Subject<String> get emojiCode => has((r) => r.emojiCode, 'emojiCode');
   Subject<ReactionType> get reactionType => has((r) => r.reactionType, 'reactionType');

test/api/route/messages_test.dart

@@ -11,7 +11,6 @@ import 'package:zulip/model/narrow.dart';
 import '../../example_data.dart' as eg;
 import '../../stdlib_checks.dart';
 import '../fake_api.dart';
-import '../model/model_checks.dart';
 import 'route_checks.dart';
 
 void main() {

test/stdlib_checks.dart

@@ -19,6 +19,73 @@ extension NullableMapChecks<K, V> on Subject<Map<K, V>?> {
   }
 }
 
+/// Convert [object] to a pure JSON-like value.
+///
+/// The result is similar to `jsonDecode(jsonEncode(object))`, but without
+/// passing through a serialized form.
+///
+/// All JSON atoms (numbers, booleans, null, and strings) are used directly.
+/// All JSON containers (lists, and maps with string keys) are copied
+/// as their elements are converted recursively.
+/// For any other value, a dynamic call `.toJson()` is made and
+/// should return either a JSON atom or a JSON container.
+Object? deepToJson(Object? object) {
+  // Implementation is based on the recursion underlying [jsonEncode],
+  // at [_JsonStringifier.writeObject] in the stdlib's convert/json.dart .
+  // (We leave out the cycle-checking, for simplicity / out of laziness.)
+
+  var (result, success) = _deeplyConvertShallowJsonValue(object);
+  if (success) return result;
+
+  final Object? shallowlyConverted;
+  try {
+    shallowlyConverted = (object as dynamic).toJson();
+  } catch (e) {
+    throw JsonUnsupportedObjectError(object, cause: e);
+  }
+
+  (result, success) = _deeplyConvertShallowJsonValue(shallowlyConverted);
+  if (success) return result;
+  throw JsonUnsupportedObjectError(object);
+}
+
+(Object? result, bool success) _deeplyConvertShallowJsonValue(Object? object) {
+  final Object? result;
+  switch (object) {
+    case null || bool() || String() || num():
+      result = object;
+    case List():
+      result = object.map((x) => deepToJson(x)).toList();
+    case Map() when object.keys.every((k) => k is String):
+      result = object.map((k, v) => MapEntry(k, deepToJson(v)));
+    default:
+      return (null, false);
+  }
+  return (result, true);
+}
+
+extension JsonChecks on Subject<Object?> {
+  /// Expects that the value is deeply equal to [expected],
+  /// after calling [deepToJson] on both.
+  ///
+  /// Deep equality is computed by [MapChecks.deepEquals]
+  /// or [IterableChecks.deepEquals].
+  void jsonEquals(Object? expected) {
+    final expectedJson = deepToJson(expected);
+    final actualJson = has((e) => deepToJson(e), 'deepToJson');
+    switch (expectedJson) {
+      case null || bool() || String() || num():
+        return actualJson.equals(expectedJson);
+      case List():
+        return actualJson.isA<List>().deepEquals(expectedJson);
+      case Map():
+        return actualJson.isA<Map>().deepEquals(expectedJson);
+      case _:
+        assert(false);
+    }
+  }
+}
+
 extension UriChecks on Subject<Uri> {
   Subject<String> get asString => has((u) => u.toString(), 'toString'); // TODO(checks): what's a good convention for this?