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

sdk/lib/_internal/compiler/implementation/compiler.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 dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 133 matching lines @@
   void registerStackTraceInCatch() {}
   void registerIsCheck(DartType type, Enqueuer enqueuer) {}
   void registerAsCheck(DartType type) {}
   void registerThrowNoSuchMethod() {}
   void registerThrowRuntimeError() {}
   void registerAbstractClassInstantiation() {}
   void registerFallThroughError() {}
   void registerSuperNoSuchMethod() {}
   void registerConstantMap() {}
   void registerRuntimeType() {}
+
+  bool isNullImplementation(ClassElement cls) {

[kasperl, 2013/03/06 08:32:26]

Why do we need this on both the compiler and the backend? Is this actually used
from anywhere?

[ngeoffray, 2013/03/06 08:40:06]

This is the backend super class, not the compiler class :-)

[kasperl, 2013/03/06 08:41:07]

Ah. Funky.

+    return cls == compiler.nullClass;
+  }
 }
 
 /**
  * Key class used in [TokenMap] in which the hash code for a token is based
  * on the [charOffset].
  */
 class TokenKey {
   final Token token;
   TokenKey(this.token);
   int get hashCode => token.charOffset;
@@ skipping 1003 matching lines @@
 
   void close() {}
 
   toString() => name;
 
   /// Convenience method for getting an [api.CompilerOutputProvider].
   static NullSink outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 }