Adding Element.created constructor

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 98bef7499846167d315e097a6bc151d9fc883f6e..2912a2efbaed10aca8810cfdfcecc94eff7291f6 100644
--- a/pkg/custom_element/lib/custom_element.dart
+++ b/pkg/custom_element/lib/custom_element.dart
@@ -171,6 +171,9 @@ class CustomElement implements Element {
    * Note that [root] will be a [ShadowRoot] if the browser supports Shadow DOM.
    */
   void created() {}
+  // Added for analyzer warnings
+  @deprecated
+  void createdCallback() {}
 
   /** Invoked when this component gets inserted in the DOM tree. */
   void inserted() {}