Add another type JSPositiveInt to show a range analysis in the inferrer would be very beneficial :-…

sdk/lib/_internal/compiler/implementation/inferrer/inferrer_visitor.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.
 
 library inferrer_visitor;
 
 import '../dart2jslib.dart' hide Selector, TypedSelector;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../tree/tree.dart';
 import '../universe/universe.dart';
 import '../util/util.dart';
 import '../types/types.dart' show TypeMask;
 
 /**
  * The interface [InferrerVisitor] will use when working on types.
  */
 abstract class TypeSystem<T> {
   T get dynamicType;
   T get nullType;
   T get intType;
   T get uint31Type;
   T get uint32Type;
+  T get positiveIntType;
   T get doubleType;
   T get numType;
   T get boolType;
   T get functionType;
   T get listType;
   T get constListType;
   T get fixedListType;
   T get growableListType;
   T get mapType;
   T get constMapType;
@@ skipping 1114 matching lines @@
     return type;
   }
 
   T visitCascade(Cascade node) {
     // Ignore the result of the cascade send and return the type of the cascade
     // receiver.
     visit(node.expression);
     return cascadeReceiverStack.removeLast();
   }
 }