Don't transform interceptors into dynamic invocations for null.

tests/language/null_to_string2_test.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 // Test correct handling of NULL object in invocation and implicit closures.
 
+class A {}
+
+A foo() => null;
+
 main() {
-  var nullObj = null;
+  A nullObj = foo();
   var x = nullObj.toString();
   Expect.isTrue(x is String);
   var y = nullObj.toString;
   Expect.isNotNull(y);
 }