Ensure UnionTypeMask.operator== works in the presence of nullability.

tests/compiler/dart2js/union_type_test.dart

Index: tests/compiler/dart2js/union_type_test.dart
===================================================================
--- tests/compiler/dart2js/union_type_test.dart	(revision 0)
+++ tests/compiler/dart2js/union_type_test.dart	(revision 0)
@@ -0,0 +1,24 @@
+// 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";
+import "parser_helper.dart";
+
+import "../../../sdk/lib/_internal/compiler/implementation/types/types.dart";
+import "../../../sdk/lib/_internal/compiler/implementation/dart_types.dart";
+
+main() {
+  Compiler compiler = new MockCompiler();
+  compiler.intClass.ensureResolved(compiler);
+  compiler.stringClass.ensureResolved(compiler);
+
+  FlatTypeMask mask1 =
+      new FlatTypeMask.exact(new InterfaceType(compiler.intClass));
+  FlatTypeMask mask2 =
+      new FlatTypeMask.exact(new InterfaceType(compiler.stringClass));
+  UnionTypeMask union1 = mask1.nonNullable().union(mask2, compiler);
+  UnionTypeMask union2 = mask2.nonNullable().union(mask1, compiler);
+  Expect.equals(union1, union2);
+}