Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/simple_inferrer_final_field3_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.
 
 // Test that we are analyzing field parameters correctly.
 
 import 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
 
@@ skipping 13 matching lines @@
 
 void main() {
   Uri uri = new Uri.fromComponents(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   compiler.runCompiler(uri);
   var typesInferrer = compiler.typesTask.typesInferrer;
 
   checkFieldTypeInClass(String className, String fieldName, type) {
     var cls = findElement(compiler, className);
     var element = cls.lookupLocalMember(buildSourceString(fieldName));
-    Expect.equals(type, typesInferrer.internal.typeOf[element]);
+    Expect.equals(type,
+        typesInferrer.internal.typeOf[element].simplify(compiler));
   }
 
   checkFieldTypeInClass('A', 'dynamicField',
       findTypeMask(compiler, 'Interceptor', 'nonNullSubclass'));
 }