Add type Event ot postEvent

IntelliJ expects a "type" field for events; otherwise the JSON will not be parsed correctly. This came up as part of making the widget rebuild table in DevTools clickable (flutter/devtools#4564)

I'm not totally sure this is the change I need, as I don't know how to test this out end-to-end (build Dart SDK and put into Flutter SDK, then run an app with these changes in DDS?). I think it's right because I'm modifying a spot that was first added in https://dart-review.googlesource.com/c/sdk/+/304981, which added `postEvent` in dds_service_extension.

Change-Id: I9a78903dca7855896b6a9b9c3f9e1d62dcb7099e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/368202
Reviewed-by: Dan Chevalier <danchevalier@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

pkg/dds/CHANGELOG.md

@@ -1,3 +1,6 @@
+# 4.2.4
+- Added missing type to `Event` in `postEvent`.
+
 # 4.2.3
 - Added missing await of `WebSocketChannel.ready` in `startDartDevelopmentService`.
 

pkg/dds/lib/src/stream_manager.dart

@@ -209,6 +209,7 @@ class StreamManager {
     streamNotify(stream, <String, dynamic>{
       'streamId': stream,
       'event': {
+        'type': 'Event',
         'kind': 'Extension',
         'timestamp': DateTime.now().millisecondsSinceEpoch,
         'extensionData': eventData,

pkg/dds/pubspec.yaml

@@ -1,5 +1,5 @@
 name: dds
-version: 4.2.3
+version: 4.2.4
 description: >-
   A library used to spawn the Dart Developer Service, used to communicate with
   a Dart VM Service instance.

pkg/dds/test/client_post_event_test.dart

@@ -62,15 +62,18 @@ void main() {
     // Test when the stream exists that the event is propagated.
     final completer = Completer<void>();
     ExtensionData? eventExtensionData;
+    Event? finalEvent;
 
     service.onEvent('testStream').listen((event) {
+      finalEvent = event;
       eventExtensionData = event.extensionData;
       completer.complete();
     });
     await service.streamListen('testStream');
     await service.postEvent('testStream', 'testKind', originalExtensionData);
 
     await completer.future;
+    expect(finalEvent?.type, equals('Event'));
     expect(eventExtensionData?.data, equals(originalExtensionData));
   });
 }