Fix receiver refining optimization in the presence of null: any Object method can be called on the …

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
===================================================================
--- tests/compiler/dart2js/mock_compiler.dart	(revision 26345)
+++ tests/compiler/dart2js/mock_compiler.dart	(working copy)
@@ -154,6 +154,8 @@
   class JSNull extends Interceptor {
     bool operator==(other) => identical(null, other);
     get hashCode => throw "JSNull.hashCode not implemented.";
+    String toString() => 'Null';
+    Type get runtimeType => null;
   }
   class JSBool extends Interceptor implements bool {
   }