Tweak Resolver driver sample to prevent warning from new analyzer.

pkg/analyzer_experimental/example/resolver_driver.dart

 #!/usr/bin/env dart
 
 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer_experimental/src/generated/java_io.dart';
 import 'package:analyzer_experimental/src/generated/source_io.dart';
 import 'package:analyzer_experimental/src/generated/ast.dart';
 import 'package:analyzer_experimental/src/generated/sdk.dart';
@@ skipping 26 matching lines @@
 
   CompilationUnit resolvedUnit = context.resolveCompilationUnit(source, libElement);
   var visitor = new _ASTVisitor();
   resolvedUnit.accept(visitor);
 }
 
 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;
         if (library != null) {
           text += " from ${element.library.definingCompilationUnit.source.fullName}";
         }
       }
     }
     print(text);
     return super.visitNode(node);
   }
 }