A selector whose type mask is nullable, applies to methods on the null implementation.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 typedef void Recompile(Element element);
 
 class ReturnInfo {
   HType returnType;
@@ skipping 1484 matching lines @@
    * into the worklist.
    *
    * Invariant: [element] must be a declaration element.
    */
   void eagerRecompile(Element element) {
     assert(invariant(element, element.isDeclaration));
     generatedCode.remove(element);
     generatedBailoutCode.remove(element);
     compiler.enqueuer.codegen.addToWorkList(element);
   }
+
+  bool isNullImplementation(ClassElement cls) {
+    return cls == jsNullClass;
+  }
 }