use VariableElement instead of VariableElementImpl in _isStateless

test/codegen/expect/cascade.js

 var cascade;
 (function(exports) {
   'use strict';
   class A extends core.Object {
     A() {
       this.x = null;
     }
   }
   // Function test_closure_with_mutate: () → void
   function test_closure_with_mutate() {
@@ skipping 53 matching lines @@
   }
   // Function test_VariableDeclaration_first: () → void
   function test_VariableDeclaration_first() {
     let a = ((_) => {
       _.length = 2;
       _.add(3);
       return _;
     })(new core.List.from([])), b = 2;
     core.print(a);
   }
+  let Base$ = dart.generic(function(T) {
+    class Base extends core.Object {
+      Base() {
+        this.x = new core.List$(T).from([]);
+      }
+    }
+    return Base;
+  });
+  let Base = Base$();
+  class Foo extends Base$(core.int) {
+    test_final_field_generic(t) {
+      this.x.add(1);
+      this.x.add(2);
+      this.x.add(3);
+      this.x.add(4);
+    }
+  }
   // Exports:
   exports.A = A;
   exports.test_closure_with_mutate = test_closure_with_mutate;
   exports.test_closure_without_mutate = test_closure_without_mutate;
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
+  exports.Base$ = Base$;
+  exports.Base = Base;
+  exports.Foo = Foo;
 })(cascade || (cascade = {}));