Handle const Symbol(...) in kernel_impact

tests/compiler/dart2js/kernel/closed_world2_test.dart

 // Copyright (c) 2017, 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.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.closed_world2_test;
 
 import 'dart:async';
 
@@ skipping 68 matching lines @@
 
 @NoInline()
 main() {
   print('Hello World');
   ''.contains; // Trigger member closurization.
   new Element.div();
   new ClassWithSetter().setter = null;
   new Class1().method1();
   new Class2().method2();
   new Class2().method3();
-  null is List<int>;
+  null is List<int>; // Use generic test
   method1(); // Both top level and instance method named 'method1' are live.
   #main; // Use a const symbol.
-  new Int8List(0);
+  new Int8List(0); // Use redirect factory to abstract native class
 }
 '''
 };
 
 main(List<String> args) {
   asyncTest(() async {
     await mainInternal(args);
   });
 }
 
@@ skipping 174 matching lines @@
 
       classes2.add(cls2);
     });
     env2.forEachClass(lib2, (ClassEntity cls2) {
       Expect.isTrue(classes2.contains(cls2), "Extra class $cls2 in $lib2");
     });
   });
 
   // TODO(johnniwinther): Test the remaining properties of [ElementEnvironment].
 }