api: Add stream events (create and delete; not yet update)

(Handling `op: update` events is zulip#182, as mentioned in the TODOs.)

Author: chrisbobbe

lib/api/model/events.dart

@@ -26,6 +26,14 @@ abstract class Event {
           default: return UnexpectedEvent.fromJson(json);
         }
       case 'message': return MessageEvent.fromJson(json);
+      case 'stream':
+        final op = json['op'] as String;
+        switch (op) {
+          case 'create': return StreamCreateEvent.fromJson(json);
+          case 'delete': return StreamDeleteEvent.fromJson(json);
+          // TODO(#182): case 'update': …
+          default: return UnexpectedEvent.fromJson(json);
+        }
       case 'heartbeat': return HeartbeatEvent.fromJson(json);
       // TODO add many more event types
       default: return UnexpectedEvent.fromJson(json);
@@ -215,6 +223,54 @@ class MessageEvent extends Event {
   }
 }
 
+/// A Zulip event of type `stream`.
+abstract class StreamEvent extends Event {
+  @override
+  @JsonKey(includeToJson: true)
+  String get type => 'stream';
+
+  String get op;
+
+  StreamEvent({required super.id});
+}
+
+/// A [StreamEvent] with op `create`: https://zulip.com/api/get-events#stream-create
+@JsonSerializable(fieldRename: FieldRename.snake)
+class StreamCreateEvent extends StreamEvent {
+  @override
+  String get op => 'create';
+
+  final List<ZulipStream> streams;
+
+  StreamCreateEvent({required super.id, required this.streams});
+
+  factory StreamCreateEvent.fromJson(Map<String, dynamic> json) =>
+    _$StreamCreateEventFromJson(json);
+
+  @override
+  Map<String, dynamic> toJson() => _$StreamCreateEventToJson(this);
+}
+
+/// A [StreamEvent] with op `delete`: https://zulip.com/api/get-events#stream-delete
+@JsonSerializable(fieldRename: FieldRename.snake)
+class StreamDeleteEvent extends StreamEvent {
+  @override
+  String get op => 'delete';
+
+  final List<ZulipStream> streams;
+
+  StreamDeleteEvent({required super.id, required this.streams});
+
+  factory StreamDeleteEvent.fromJson(Map<String, dynamic> json) =>
+    _$StreamDeleteEventFromJson(json);
+
+  @override
+  Map<String, dynamic> toJson() => _$StreamDeleteEventToJson(this);
+}
+
+// TODO(#182) StreamUpdateEvent, for a [StreamEvent] with op `update`:
+//   https://zulip.com/api/get-events#stream-update
+
 @JsonSerializable(fieldRename: FieldRename.snake)
 class HeartbeatEvent extends Event {
   @override