Add UInt32 and UInt31 types to better infer bit operations.

tests/compiler/dart2js/list_tracer2_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.
 
 // We used to always nullify the element type of a list we are tracing in
 // the presence of a fixed length list constructor call.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import
@@ skipping 17 matching lines @@
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesInferrer = compiler.typesTask.typesInferrer;
 
     checkType(String name, type) {
       var element = findElement(compiler, name);
       ContainerTypeMask mask = typesInferrer.getTypeOfElement(element);
       Expect.equals(type, mask.elementType.simplify(compiler), name);
     }
 
-    checkType('myList', compiler.typesTask.intType);
+    checkType('myList', compiler.typesTask.uint31Type);
   }));
 }