Fix for dartbug.com/6036: the intersection of two different types is not always conflicting.

tests/language/typed_equality_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.
+
+// Regression test for http://dartbug.com/6036. dart2js used to fail
+// this method because it was computing that intersecting type D with
+// type C is conflicting.
+
+foo(a, b) {
+  if (a === b) return;
+  throw 'broken';
+}
+
+class D {}
+
+class C implements D {}
+
+main() {
+  var c = new C();
+  foo(c, c as D);
+}