for discussion: support for zones in DOM events

tools/dom/src/EventStreamProvider.dart

Index: tools/dom/src/EventStreamProvider.dart
diff --git a/tools/dom/src/EventStreamProvider.dart b/tools/dom/src/EventStreamProvider.dart
index a9c293da5da7083b938ffbd80785bdea4cf32068..41d8f3469bba3ccd7be1f76d9b48c170254d5093 100644
--- a/tools/dom/src/EventStreamProvider.dart
+++ b/tools/dom/src/EventStreamProvider.dart
@@ -154,11 +154,20 @@ class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
   var _onData;
   final bool _useCapture;
 
-  _EventStreamSubscription(this._target, this._eventType, this._onData,
-      this._useCapture) {
+  _EventStreamSubscription(this._target, this._eventType, onData,
+      this._useCapture) : _onData = _wrapZone(onData) {
     _tryResume();
   }
 
+  static _wrapZone(callback) {
+    // For performance reasons, avoid wrapping if we are in the default zone.
+    final zone = currentZoneExperimental;
+    if (zone.isDefault) return callback;
+
+    zone.expectCallback();
+    return (e) => zone.executePeriodicCallbackGuarded(() => callback(e));
+  }
+
   void cancel() {
     if (_canceled) return;
 
@@ -166,6 +175,9 @@ class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
     // Clear out the target to indicate this is complete.
     _target = null;
     _onData = null;
+
+    final zone = currentZoneExperimental;
+    if (!zone.isDefault) zone.cancelCallbackExpectation();
   }
 
   bool get _canceled => _target == null;
@@ -177,7 +189,7 @@ class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
     // Remove current event listener.
     _unlisten();
 
-    _onData = handleData;
+    _onData = _wrapZone(handleData);
     _tryResume();
   }