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

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 708bdb016ca2a9c401377247d8acbd598516f465..a6f71bceeb746e7329aa4dc9d7c5ee8cc2e772d9 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -11063,7 +11063,33 @@ class Event extends Interceptor native "Event" {
     return e;
   }
 
-  Event._private();
+  /** 
+   * Caches the pointer to the element that matches CSS selector involved in
+   * the event.
+   */
+  Element _cachedMatchingTarget;
+
+  /**
+   * 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 (target.matches('.selector')) {
+          matchedTarget = target;
+        }
+        target = target.parent;
+      }
+      _cachedMatchingTarget = matchedTarget;
+    }
+    return _cachedMatchingTarget;
+  }
 
   @DomName('Event.AT_TARGET')
   @DocsEditable()