Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/call_site_type_inferer_test.dart

Index: tests/compiler/dart2js/call_site_type_inferer_test.dart
===================================================================
--- tests/compiler/dart2js/call_site_type_inferer_test.dart	(revision 23010)
+++ tests/compiler/dart2js/call_site_type_inferer_test.dart	(working copy)
@@ -228,7 +228,8 @@
             .toList();
         Expect.isFalse(types.allUnknown);
         Expect.equals(expectedTypes.length, types.types.length);
-        Expect.listEquals(expectedTypes, types.types);
+        Expect.listEquals(expectedTypes,
+            types.types.map((t) => t.simplify(compiler)).toList());
       } else {
         Expect.isTrue(types.allUnknown);
       }