Add UInt32 and UInt31 types to better infer bit operations.

tests/compiler/dart2js/simple_inferrer_unregister_call_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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
 
 const String TEST = """
@@ skipping 23 matching lines @@
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesInferrer = compiler.typesTask.typesInferrer;
 
     checkReturnInClass(String className, String methodName, type) {
       var cls = findElement(compiler, className);
       var element = cls.lookupLocalMember(methodName);
       Expect.equals(type, typesInferrer.getReturnTypeOfElement(element));
     }
 
-    checkReturnInClass('A', '+', compiler.typesTask.intType);
+    checkReturnInClass('A', '+', compiler.typesTask.uint31Type);
   }));
 }