Deprecate MouseEvent.elementTo and elementFrom since they have the same

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 ae483932b4b0e0dd431d06d7e081a634928ac563..56326f3bc3dcdb9d32b783f366140519a18fd3fb 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -19154,9 +19154,16 @@ class MouseEvent extends UIEvent {
   @Unstable()
   final DataTransfer dataTransfer;
 
+  /**
+   * The nonstandard way to access the element that the mouse comes
+   * from in the case of a `mouseover` event.
+   *
+   * This member is deprecated and not cross-browser compatible; use
+   * relatedTarget to get the same information in the standard way.
+   */
   @DomName('MouseEvent.fromElement')
   @DocsEditable()
-  @Experimental() // nonstandard
+  @deprecated
   final Node fromElement;
 
   @DomName('MouseEvent.metaKey')
@@ -19187,9 +19194,16 @@ class MouseEvent extends UIEvent {
   @DocsEditable()
   final bool shiftKey;
 
+  /**
+   * The nonstandard way to access the element that the mouse goes
+   * to in the case of a `mouseout` event.
+   *
+   * This member is deprecated and not cross-browser compatible; use
+   * relatedTarget to get the same information in the standard way.
+   */
   @DomName('MouseEvent.toElement')
   @DocsEditable()
-  @Experimental() // nonstandard
+  @deprecated
   final Node toElement;
 
   @JSName('webkitMovementX')