Tweak Resolver driver sample to prevent warning from new analyzer.

pkg/analyzer_experimental/example/resolver_driver.dart

Index: pkg/analyzer_experimental/example/resolver_driver.dart
diff --git a/pkg/analyzer_experimental/example/resolver_driver.dart b/pkg/analyzer_experimental/example/resolver_driver.dart
index b3261b78206119b6a5822045458f95c13df3c959..d28d9644265800de2886bb19f6ba2d6c1d49a7a9 100644
--- a/pkg/analyzer_experimental/example/resolver_driver.dart
+++ b/pkg/analyzer_experimental/example/resolver_driver.dart
@@ -44,7 +44,7 @@ class _ASTVisitor extends GeneralizingASTVisitor {
   visitNode(ASTNode node) {
     String text = '${node.runtimeType} : <"${node.toString()}">';
     if (node is SimpleIdentifier) {
-      Element element = node.element;
+      Element element = (node as SimpleIdentifier).element;
       if (element != null) {
         text += " element: ${element.runtimeType}";
         LibraryElement library = element.library;