Adding missing Element APIs to CustomElement, fixing analyzer errors.

pkg/custom_element/lib/custom_element.dart

Index: pkg/custom_element/lib/custom_element.dart
diff --git a/pkg/custom_element/lib/custom_element.dart b/pkg/custom_element/lib/custom_element.dart
index 50cc1a6e373fdddfe85bbc85a2b0f0b5d67c6488..aa540e5c7a216f015f2f3671914982bca97b203f 100644
--- a/pkg/custom_element/lib/custom_element.dart
+++ b/pkg/custom_element/lib/custom_element.dart
@@ -591,6 +591,8 @@ class CustomElement implements Element {
   Stream<KeyboardEvent> get onKeyUp => host.onKeyUp;
   Stream<Event> get onLoad => host.onLoad;
   Stream<MouseEvent> get onMouseDown => host.onMouseDown;
+  Stream<MouseEvent> get onMouseEnter => host.onMouseEnter;
+  Stream<MouseEvent> get onMouseLeave => host.onMouseLeave;
   Stream<MouseEvent> get onMouseMove => host.onMouseMove;
   Stream<Event> get onFullscreenChange => host.onFullscreenChange;
   Stream<Event> get onFullscreenError => host.onFullscreenError;