remove connections faster

pkgs/sse/lib/server/sse_handler.dart

diff --git a/pkgs/sse/lib/server/sse_handler.dart b/pkgs/sse/lib/server/sse_handler.dart
index 9c1f57f..0c06a14 100644
--- a/pkgs/sse/lib/server/sse_handler.dart
+++ b/pkgs/sse/lib/server/sse_handler.dart
@@ -7,6 +7,7 @@
 
 import 'package:async/async.dart';
 import 'package:logging/logging.dart';
+import 'package:pedantic/pedantic.dart';
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:stream_channel/stream_channel.dart';
 
@@ -28,11 +29,11 @@
   final Sink _sink;
   final String _clientId;
 
-  var _isClosed = false;
+  final _closedCompleter = Completer<void>();
 
   SseConnection(this._sink, this._clientId) {
     _outgoingController.stream.listen((data) {
-      if (!_isClosed) {
+      if (!_closedCompleter.isCompleted) {
         // JSON encode the message to escape new lines.
         _sink.add('data: ${json.encode(data)}\n');
         _sink.add('\n');
@@ -54,8 +55,8 @@
   Stream<String> get stream => _incomingController.stream;
 
   void _close() {
-    if (!_isClosed) {
-      _isClosed = true;
+    if (!_closedCompleter.isCompleted) {
+      _closedCompleter.complete();
       _sink.close();
       if (!_outgoingController.isClosed) _outgoingController.close();
       if (!_incomingController.isClosed) _incomingController.close();
@@ -103,14 +104,15 @@
       var clientId = req.url.queryParameters['sseClientId'];
       var connection = SseConnection(sink, clientId);
       _connections[clientId] = connection;
+      unawaited(connection._closedCompleter.future.then((_) {
+        _connections.remove(clientId);
+      }));
       // Remove connection when it is remotely closed or the stream is
       // cancelled.
       channel.stream.listen((_) {
         // SSE is unidirectional. Responses are handled through POST requests.
       }, onDone: () {
-        // Trigger closing the connection.
-        connection.sink.close();
-        _connections.remove(clientId);
+        connection._close();
       });
 
       _connectionController.add(connection);

pkgs/sse/pubspec.yaml

diff --git a/pkgs/sse/pubspec.yaml b/pkgs/sse/pubspec.yaml
index e2a5c98..054332b 100644
--- a/pkgs/sse/pubspec.yaml
+++ b/pkgs/sse/pubspec.yaml
@@ -14,6 +14,7 @@
   async: ^2.0.8
   http: ^0.12.0+1
   logging: ^0.11.3+2
+  pedantic: ^1.4.0
   stream_channel: ^1.6.8
   shelf: ^0.7.4
   uuid: ^1.0.3