use VariableElement instead of VariableElementImpl in _isStateless

test/codegen/cascade.dart

 // Copyright (c) 2015, 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.
 
 class A {
   var x;
 }
 
 void test_closure_with_mutate() {
   var a = new A();
@@ skipping 50 matching lines @@
   print(b);
 }
 
 void test_VariableDeclaration_first() {
   var a = []
     ..length = 2
     ..add(3),
       b = 2;
   print(a);
 }
+
+class Base<T> {
+  final List<T> x = <T>[];
+}
+
+class Foo extends Base<int> {
+  void test_final_field_generic(t) {
+    x..add(1)..add(2)..add(3)..add(4);
+  }
+}