Adding missing Element APIs to CustomElement, fixing analyzer errors.

pkg/custom_element/lib/custom_element.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Custom Elements let authors define their own elements. Authors associate code
  * with custom tag names, and then use those custom tag names as they would any
  * standard tag. See <www.polymer-project.org/platform/custom-elements.html>
  * for more information.
  */
@@ skipping 573 matching lines @@
   Stream<MouseEvent> get onDrop => host.onDrop;
   Stream<Event> get onError => host.onError;
   Stream<Event> get onFocus => host.onFocus;
   Stream<Event> get onInput => host.onInput;
   Stream<Event> get onInvalid => host.onInvalid;
   Stream<KeyboardEvent> get onKeyDown => host.onKeyDown;
   Stream<KeyboardEvent> get onKeyPress => host.onKeyPress;
   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;
   Stream<Event> get onPaste => host.onPaste;
   Stream<Event> get onReset => host.onReset;
   Stream<Event> get onScroll => host.onScroll;
   Stream<Event> get onSearch => host.onSearch;
   Stream<Event> get onSelect => host.onSelect;
   Stream<Event> get onSelectStart => host.onSelectStart;
   Stream<Event> get onSubmit => host.onSubmit;
@@ skipping 50 matching lines @@
       for (var removed in record.removedNodes) {
         if (identical(node, removed)) {
           observer.disconnect();
           element.removed();
           return;
         }
       }
     }
   }).observe(element.parentNode, childList: true);
 }