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

tools/dom/templates/html/impl/impl_Event.darttemplate

 // Copyright (c) 2012, 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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   // In JS, canBubble and cancelable are technically required parameters to
@@ skipping 15 matching lines @@
    * Normally events should be created via their constructors, if available.
    *
    *     var e = new Event.type('MouseEvent', 'mousedown', true, true);
    */
   factory Event.eventType(String type, String name, {bool canBubble: true,
       bool cancelable: true}) {
     final Event e = document._createEvent(type);
     e._initEvent(name, canBubble, cancelable);
     return e;
   }
+  
+  /** The CSS selector involved with event delegation. */
+  String _selector;
 
-  $CLASSNAME._private();
+  /**
+   * 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.

[blois, 2013/10/09 23:12:57]

Update comment for exception

+   */
+  Element matchingTarget() {
+    if (_selector == null) {
+      throw new UnsupportedError('Cannot call matchingTarget if this Event did'
+          ' not arise as a result of event delegation.');
+    }
+    var currentTarget = this.currentTarget;
+    var target = this.target;
+    var matchedTarget;
+    while (matchedTarget == null && target != currentTarget && target != null) {
+      if (target.matches(_selector)) {
+        matchedTarget = target;
+      }
+      target = target.parent;
+    }
+    return matchedTarget;
+  }
 $!MEMBERS
 }