[dart:html] rename custom elements callbacks to match spec

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 4155c8d849190d06c1c52fb44702c23a3dc84612..0005e02ffab1912d6a13580f7b92300de8601ad8 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -738,15 +738,39 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   /**
    * Called by the DOM when this element has been inserted into the live
    * document.
+   *
+   * More information can be found in the
+   * [Custom Elements](http://w3c.github.io/webcomponents/spec/custom/#dfn-attached-callback)
+   * draft specification.
    */
   @Experimental()
-  void enteredView() {}
+  void attached() {
+    // For the deprecation period, call the old callback.
+    enteredView();
+  }
 
   /**
    * Called by the DOM when this element has been removed from the live
    * document.
+   *
+   * More information can be found in the
+   * [Custom Elements](http://w3c.github.io/webcomponents/spec/custom/#dfn-detached-callback)
+   * draft specification.
    */
   @Experimental()
+  void detached() {
+    // For the deprecation period, call the old callback.
+    leftView();
+  }
+
+  /** *Deprecated*: override [attached] instead. */
+  @Experimental()
+  @deprecated
+  void enteredView() {}
+
+  /** *Deprecated*: override [detached] instead. */
+  @Experimental()
+  @deprecated
   void leftView() {}
 
   /**