Fix issue 6472 that exhibited a few crashing bugs in dart2js.

tests/language/checked_null_test.dart

+// 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.
+
+class A {
+  Map a;
+  Comparator b;
+  // This code exhibited a bug in dart2js checked mode, where the type
+  // of [a] was inferred to be [Comparator] or null;
+  A() : b = null, a = null;
+}
+
+main() { 
+  // We would create a typed selector for the call to foo, where the
+  // receiver type is a typedef. Some code in the dart2js backend were
+  // not dealing correctly with typedefs and lead the compiler to
+  // crash.
+  new A().a.foo();
+}