[element model] migrate elements_base

Change-Id: I960ffa6f44240b7b65c6d38f93f58e1f7cde6fb0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/403902
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Auto-Submit: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/analyzer_use_new_elements.txt

@@ -177,7 +177,6 @@ test/src/summary/element_text.dart
 test/src/summary/elements/metadata_test.dart
 test/src/summary/elements/top_level_variable_test.dart
 test/src/summary/elements/type_inference_test.dart
-test/src/summary/elements_base.dart
 test/src/summary/macro_test.dart
 test/src/summary/resolved_ast_printer.dart
 test/src/task/strong/dart2_inference_test.dart

pkg/analyzer/test/src/summary/elements_base.dart

@@ -4,6 +4,7 @@
 
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/file_system/file_system.dart';
+import 'package:analyzer/src/dart/analysis/results.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ -68,6 +69,6 @@ abstract class ElementsBaseTest extends PubPackageResolutionTest {
 
 extension on SomeLibraryElementResult {
   LibraryElementImpl get element {
-    return (this as LibraryElementResult).element as LibraryElementImpl;
+    return (this as LibraryElementResultImpl).element2;
   }
 }