Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/field_type_inferer_test.dart

Index: tests/compiler/dart2js/field_type_inferer_test.dart
===================================================================
--- tests/compiler/dart2js/field_type_inferer_test.dart	(revision 23010)
+++ tests/compiler/dart2js/field_type_inferer_test.dart	(working copy)
@@ -344,7 +344,8 @@
       name,
       disableInlining,
       (backend, field) {
-        HType inferredType = backend.optimisticFieldType(field);
+        HType inferredType =
+            backend.optimisticFieldType(field).simplify(backend.compiler);
         if (type is Function) type = type(backend.compiler);
         Expect.equals(type, inferredType);
     });