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

sdk/lib/_internal/compiler/implementation/universe/universe.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.
 
 library universe;
 
 import '../closure.dart';
 import '../elements/elements.dart';
 import '../dart2jslib.dart';
 import '../dart_types.dart';
@@ skipping 489 matching lines @@
 
     // TODO(kasperl): Can't we just avoid creating typed selectors
     // based of function types?
     Element self = mask.base.element;
     if (self.isTypedef()) {
       // A typedef is a function type that doesn't have any
       // user-defined members.
       return false;
     }
 
-    if (mask.isExact) {
+    if (mask.isNullable && compiler.backend.isNullImplementation(other)) {
+      return appliesUntyped(element, compiler);
+    } else if (mask.isExact) {
       return hasElementIn(self, element) && appliesUntyped(element, compiler);
     } else if (mask.isSubclass) {
       return (hasElementIn(self, element)
               || other.isSubclassOf(self)
               || compiler.world.hasAnySubclassThatMixes(self, other))
           && appliesUntyped(element, compiler);
     } else {
       assert(mask.isSubtype);
       if (other.implementsInterface(self)
           || other.isSubclassOf(self)
           || compiler.world.hasAnySubclassThatMixes(self, other)
           || compiler.world.hasAnySubclassThatImplements(other, mask.base)) {
         return appliesUntyped(element, compiler);
       }
 
       // If [self] is a subclass of [other], it inherits the
       // implementation of [element].
       ClassElement cls = self;
       if (cls.isSubclassOf(other)) {
         // Resolve an invocation of [element.name] on [self]. If it
         // is found, this selector is a candidate.
         return hasElementIn(cls, element) && appliesUntyped(element, compiler);
       }
     }
     return false;
   }
 }