Revert "Revert "Dart2js support for custom element lifecycle events""

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 8f73476b88f76d38cd1c7ff3a32f1c7ef6fe064b..5732c1ebd5469464a8cdc917a75156394a97d705 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -680,6 +680,20 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   @Experimental()
   void created() {}
 
+  /**
+   * Called by the DOM when this element has been inserted into the live
+   * document.
+   */
+  @Experimental()
+  void enteredView() {}
+
+  /**
+   * Called by the DOM when this element has been removed from the live
+   * document.
+   */
+  @Experimental()
+  void leftView() {}
+
   // Hooks to support custom WebComponents.
 
 $if DART2JS