Use generic functions in zones.

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 c5e929c8d17fba4ce5b87db5e2a0f7db455ede09..bb24647e458785839976dd312e7dcf178a72dce4 100644
--- a/tools/dom/src/EventStreamProvider.dart
+++ b/tools/dom/src/EventStreamProvider.dart
@@ -225,9 +225,9 @@ class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
 
   // TODO(leafp): It would be better to write this as
   // _onData = onData == null ? null :
-  //   onData is _wrapZoneCallback<Event, dynamic>
-  //     ? _wrapZone/*<Event, dynamic>*/(onData)
-  //     : _wrapZone/*<Event, dynamic>*/((e) => onData(e as T))
+  //   onData is void Function(Event)
+  //     ? _wrapZone<Event>(onData)
+  //     : _wrapZone<Event>((e) => onData(e as T))
   // In order to support existing tests which pass the wrong type of events but
   // use a more general listener, without causing as much slowdown for things
   // which are typed correctly.  But this currently runs afoul of restrictions
@@ -236,7 +236,7 @@ class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
       this._target, this._eventType, void onData(T event), this._useCapture)
       : _onData = onData == null
             ? null
-            : _wrapZone<Event, dynamic>((e) => (onData as dynamic)(e)) {
+            : _wrapZone<Event>((e) => (onData as dynamic)(e)) {
     _tryResume();
   }
 
@@ -258,7 +258,7 @@ class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
     }
     // Remove current event listener.
     _unlisten();
-    _onData = _wrapZone/*<Event, dynamic>*/(handleData);
+    _onData = _wrapZone<Event>(handleData);
     _tryResume();
   }