Detect (null type == null type), and change it to true.

tests/compiler/dart2js/dead_code_test.dart

+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+import 'compiler_helper.dart';
+
+String TEST = r'''
+main() {
+  foo(null);
+}
+foo(a) {
+  if (a != null) return 42;
+  return 54;
+}
+''';
+
+main() {
+  String generated = compileAll(TEST);
+  Expect.isFalse(generated.contains('return 42'), 'dead code not eliminated');
+}