Add UInt32 and UInt31 types to better infer bit operations.

tests/compiler/dart2js/mirror_final_field_inferrer_test.dart

Index: tests/compiler/dart2js/mirror_final_field_inferrer_test.dart
===================================================================
--- tests/compiler/dart2js/mirror_final_field_inferrer_test.dart	(revision 30666)
+++ tests/compiler/dart2js/mirror_final_field_inferrer_test.dart	(working copy)
@@ -28,7 +28,7 @@
     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');
   }));