Fix InterfaceType.treatAsRaw

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 64 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>;
 }
 '''
 };
 
 main(List<String> args) {
   asyncTest(() async {
     await mainInternal(args);
   });
 }
 
@@ skipping 264 matching lines @@
 
   MemoryDillLibraryLoaderTask(KernelToElementMap elementMap,
       DiagnosticReporter reporter, Measurer measurer, this.program)
       : super(elementMap, null, null, reporter, measurer);
 
   Future<LoadedLibraries> loadLibrary(Uri resolvedUri,
       {bool skipFileWithPartOfTag: false}) async {
     return createLoadedLibraries(program);
   }
 }