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

sdk/lib/_internal/compiler/implementation/constants.dart

 // Copyright (c) 2012, 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.
 
 part of dart2js;
 
 abstract class ConstantVisitor<R> {
   R visitFunction(FunctionConstant constant);
   R visitNull(NullConstant constant);
   R visitInt(IntConstant constant);
@@ skipping 146 matching lines @@
       case 10: return const IntConstant._internal(10);
       case -1: return const IntConstant._internal(-1);
       case -2: return const IntConstant._internal(-2);
       default: return new IntConstant._internal(value);
     }
   }
   const IntConstant._internal(this.value);
   bool isInt() => true;
   bool isUInt31() => value >= 0 && value < (1 << 31);
   bool isUInt32() => value >= 0 && value < (1 << 32);
+  bool isPositive() => value >= 0;

[kasperl, 2013/11/28 09:09:47]

Stephen would probably insist on this being isNonNegative. He's probably right
:-)

 
   DartType computeType(Compiler compiler) {
     return compiler.intClass.computeType(compiler);
   }
 
   ti.TypeMask computeMask(Compiler compiler) {
     if (isUInt31()) return compiler.typesTask.uint31Type;
     if (isUInt32()) return compiler.typesTask.uint32Type;
+    if (isPositive()) return compiler.typesTask.positiveIntType;
     return compiler.typesTask.intType;
   }
 
   // We have to override the equality operator so that ints and doubles are
   // treated as separate constants.
   // The is [:!IntConstant:] check at the beginning of the function makes sure
   // that we compare only equal to integer constants.
   bool operator ==(var other) {
     if (other is !IntConstant) return false;
     IntConstant otherInt = other;
@@ skipping 421 matching lines @@
       if (i > 0) sb.write(',');
       sb.write(Error.safeToString(field.name));
       sb.write('=');
       sb.write(Error.safeToString(value));
       i++;
     });
     sb.write('))');
     return sb.toString();
   }
 }