Make EventStreamSubscription DDC safe.

tools/dom/src/EventStreamProvider.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of html;
 
 /**
  * A factory to expose DOM events as Streams.
  */
 class EventStreamProvider<T extends Event> {
@@ skipping 208 matching lines @@
 // use generics until dartbug/26276 is fixed.
 typedef _EventListener<T extends Event>(T event);
 
 class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
   int _pauseCount = 0;
   EventTarget _target;
   final String _eventType;
   EventListener _onData;
   final bool _useCapture;
 
-  // TODO(jacobr): for full strong mode correctness we should write
-  // _onData = onData == null ? null : _wrapZone/*<Event, dynamic>*/((e) => onData(e as T))
-  // but that breaks 114 co19 tests as well as multiple html tests as it is reasonable
-  // to pass the wrong type of event object to an event listener as part of a
-  // test.
+  // 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))
+  // 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
+  // on is checks for compatibility with the VM.
   _EventStreamSubscription(this._target, this._eventType, void onData(T event),
-      this._useCapture) : _onData = _wrapZone/*<Event, dynamic>*/(onData) {
+                           this._useCapture) :
+      _onData = onData == null
+      ? null
+      : _wrapZone/*<Event, dynamic>*/((e) => (onData as dynamic)(e))
+  {
     _tryResume();
   }
 
   Future cancel() {
     if (_canceled) return null;
 
     _unlisten();
     // Clear out the target to indicate this is complete.
     _target = null;
     _onData = null;
@@ skipping 191 matching lines @@
     return new _ElementListEventStreamImpl<T>(e, _eventTypeGetter(e), useCapture);
   }
 
   String getEventType(EventTarget target) {
     return _eventTypeGetter(target);
   }
 
   String get _eventType =>
       throw new UnsupportedError('Access type through getEventType method.');
 }