Add UInt32 and UInt31 types to better infer bit operations.

tests/compiler/dart2js/mirror_final_field_inferrer_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 final fields in @MirrorsUsed are still inferred.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart' show compilerFor;
 import 'compiler_helper.dart' show findElement;
@@ skipping 10 matching lines @@
 }
 """
 };
 
 void main() {
   var compiler = compilerFor(MEMORY_SOURCE_FILES);
   asyncTest(() => compiler.runCompiler(Uri.parse('memory:main.dart')).then((_) {
     var element = findElement(compiler, 'field');
     var typesTask = compiler.typesTask;
     var typesInferrer = typesTask.typesInferrer;        
-    Expect.equals(typesTask.intType,
+    Expect.equals(typesTask.uint31Type,
                   typesInferrer.getTypeOfElement(element).simplify(compiler),
                   'field');
   }));
 }