Add UInt32 and UInt31 types to better infer bit operations.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
===================================================================
--- tests/compiler/dart2js/mock_compiler.dart	(revision 30666)
+++ tests/compiler/dart2js/mock_compiler.dart	(working copy)
@@ -122,6 +122,8 @@
     toString() {}
     operator+(other) => this;
   }
+  class JSUInt32 extends JSInt {}
+  class JSUInt31 extends JSUInt32 {}
   class JSNumber extends Interceptor implements num {
     // All these methods return a number to please type inferencing.
     operator-() => (this is JSInt) ? 42 : 42.2;