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

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 55bf8b90ae93e96a1a43d6fd1f35721ea6209177..00fd48f6772875ded7038bc61e9a393398cac7df 100644
--- a/tools/dom/src/EventStreamProvider.dart
+++ b/tools/dom/src/EventStreamProvider.dart
@@ -52,7 +52,9 @@ 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);
 }
 
 /**
@@ -74,7 +76,9 @@ class _ElementListEventStreamImpl<T extends Event> extends Stream<T>
   }
 
   Stream<T> matches(String selector) =>

[blois, 2013/10/09 22:25:44]

how about =>
this.where((event) => event.target.matchesWithAncestors(selector)).map((e) {
 e._selector = selector;
 return e;
});

Rather than adding the new class?

[Emily Fortuna, 2013/10/09 22:39:18]

Good point. fixed.

-      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),
@@ -90,6 +94,27 @@ 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.
  */