Add "matchingTarget" function to Event, which is the Element that matched the CSS value selector.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 73f50e22c232442e2b8f42fb898e6b27e526d071..f091d9548d9af5c5958862529e89e8351eeb90a6 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -11476,7 +11476,36 @@ class Event extends NativeFieldWrapperClass1 {
     return e;
   }
 
-  Event._private();
+  /** 
+   * Caches the pointer to the element that matches CSS selector involved in
+   * the event.
+   */
+  Element _cachedMatchingTarget;
+  
+  /** The CSS selector involved with event delegation. */
+  String _selector;
+
+  /**
+   * A pointer to the element whose CSS selector matched within which an event
+   * was fired. If this Event was not associated with any Event delegation,
+   * then this value will be null.
+   */
+  Element matchingTarget() {
+    if (_cachedMatchingTarget == null) {
+      var currentTarget = this.currentTarget;
+      var target = this.target;
+      var matchedTarget;
+      while (matchedTarget == null && target != currentTarget && 
+          target != null) {
+        if (_selector != null && target.matches(_selector)) {
+          matchedTarget = target;
+        }
+        target = target.parent;
+      }
+      _cachedMatchingTarget = matchedTarget;
+    }
+    return _cachedMatchingTarget;
+  }
 
   @DomName('Event.AT_TARGET')
   @DocsEditable()
@@ -31152,7 +31181,9 @@ class _ElementEventStreamImpl<T extends Event> extends _EventStream<T>
       super(target, eventType, useCapture);
 
   Stream<T> matches(String selector) =>
-      this.where((event) => event.target.matchesWithAncestors(selector));
+      new _MatchingStream(
+          this.where((event) => event.target.matchesWithAncestors(selector)),
+          selector);
 }
 
 /**
@@ -31174,7 +31205,9 @@ class _ElementListEventStreamImpl<T extends Event> extends Stream<T>
   }
 
   Stream<T> matches(String selector) =>
-      this.where((event) => event.target.matchesWithAncestors(selector));
+    new _MatchingStream(
+        this.where((event) => event.target.matchesWithAncestors(selector)),
+        selector);
 
   // Delegate all regular Stream behavor to our wrapped Stream.
   StreamSubscription<T> listen(void onData(T event),
@@ -31190,6 +31223,27 @@ class _ElementListEventStreamImpl<T extends Event> extends Stream<T>
 }
 
 /**
+ * A stream that keeps track of what CSS selectors it used to collect element
+ * events for event delegation.
+ */
+class _MatchingStream<T extends Event> extends Stream<T> {
+  Stream<T> _stream;
+  String selector;
+
+  _MatchingStream(Stream this._stream, String this.selector);
+
+  // Delegate all regular Stream behavor to our wrapped Stream.
+  StreamSubscription<T> listen(void onData(T event),
+      { void onError(error),
+        void onDone(),
+        bool cancelOnError}) =>
+      _stream.listen((e) { e._selector = selector; return onData(e); },
+          onError: onError,
+          onDone: onDone,
+          cancelOnError: cancelOnError);
+}
+
+/**
  * A stream of custom events, which enables the user to "fire" (add) their own
  * custom events to a stream.
  */