Fix type check elimination bug in code generator (issue 11792).

tests/language/malformed_type_test.dart

Index: tests/language/malformed_type_test.dart
===================================================================
--- tests/language/malformed_type_test.dart	(revision 0)
+++ tests/language/malformed_type_test.dart	(revision 0)
@@ -0,0 +1,13 @@
+// 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.
+
+// Regression test for issue 11792.
+
+null_() => null;
+final Undeclared x = null_();  // null is assignable to x of malformed type.
+
+main() {
+  print(x);
+}
+