Fully polyfill KeyEvent so that you can programmatically create your own "keyboard" events.

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 bd6e04ca6e2cffbeae0845d50360ae6f00e652f4..55bf8b90ae93e96a1a43d6fd1f35721ea6209177 100644
--- a/tools/dom/src/EventStreamProvider.dart
+++ b/tools/dom/src/EventStreamProvider.dart
@@ -90,6 +90,61 @@ class _ElementListEventStreamImpl<T extends Event> extends Stream<T>
 }
 
 /**
+ * A stream of custom events, which enables the user to "fire" (add) their own
+ * custom events to a stream.
+ */
+abstract class CustomStream<T extends Event> implements Stream<T> {
+  /**
+   * Add the following custom event to the stream for dispatching to interested
+   * listeners.
+   */
+  void add(T event);
+}
+
+class _CustomEventStreamImpl<T extends Event> extends Stream<T>
+    implements CustomStream<T> {
+  StreamController<T> _streamController;
+  /** The type of event this stream is providing (e.g. "keydown"). */
+  String _type;
+
+  _CustomEventStreamImpl(String type) {
+    _type = type;
+    _streamController = new StreamController.broadcast(sync: true);
+  }
+
+  // Delegate all regular Stream behavior to our wrapped Stream.
+  StreamSubscription<T> listen(void onData(T event),
+      { void onError(error),
+        void onDone(),
+        bool cancelOnError}) {
+    return _streamController.stream.listen(onData, onError: onError,
+        onDone: onDone, cancelOnError: cancelOnError);
+  }
+
+  Stream<T> asBroadcastStream({void onListen(StreamSubscription subscription),
+                               void onCancel(StreamSubscription subscription)})
+      => _streamController.stream;
+
+  bool get isBroadcast => true;
+
+  void add(T event) {
+    if (event.type == _type) _streamController.add(event);
+  }
+}
+
+class _CustomKeyEventStreamImpl extends _CustomEventStreamImpl<KeyEvent>
+    implements CustomStream<KeyEvent> {
+  _CustomKeyEventStreamImpl(String type) : super(type);
+
+  void add(KeyEvent event) {
+    if (event.type == _type) {
+      event.currentTarget.dispatchEvent(event._parent);
+      _streamController.add(event);
+    }
+  }
+}
+
+/**
  * A pool of streams whose events are unified and emitted through a central
  * stream.
  */