Adding event delegation.

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 776d6026eb45c8c5377458de96f3e6914cc88ad4..e147bc613bb5b09658618123440d7a1aa7c2b27d 100644
--- a/tools/dom/src/EventStreamProvider.dart
+++ b/tools/dom/src/EventStreamProvider.dart
@@ -134,6 +134,42 @@ class EventStreamProvider<T extends Event> {
   Stream<T> forTarget(EventTarget e, {bool useCapture: false}) {
     return new _EventStream(e, _eventType, useCapture);
   }
+
+  /**
+   * Gets a single event [Stream] which is the combinations of event streams
+   * for this event for each target.
+   *
+   * See also:
+   *
+   * * [forTarget]
+   */
+  Stream<T> forTargets(List<EventTarget> targets, {bool useCapture: false}) {
+    // Clone so modifications to the list don't affect this.
+    targets = new List.from(targets);
+
+    // Keep track of subscriptions so we unsubscribe from all when the stream
+    // we return is unsubscribed.
+    var subscriptions = [];
+    var controller;
+    controller = new StreamController<T>(onSubscriptionStateChange: () {
+      if (controller.hasSubscribers) {
+        assert(subscriptions.isEmpty);
+        for (var target in targets) {
+          subscriptions.add(forTarget(target, useCapture: useCapture).listen(
+            (e) {
+              controller.add(e);
+            }));
+        }
+      } else {
+        assert(subscriptions.length == targets.length);
+        for (var subscription in subscriptions) {
+          subscription.cancel();
+        }
+        subscriptions.clear();
+      }
+    });
+    return controller.stream;
+  }
 }
 
 /**