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

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

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/compiler.dart	(revision 19535)
+++ sdk/lib/_internal/compiler/implementation/compiler.dart	(working copy)
@@ -151,6 +151,10 @@
   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;
+  }
 }
 
 /**