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

sdk/lib/_internal/compiler/implementation/ssa/optimize.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(revision 30754)
+++ sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(working copy)
@@ -214,7 +214,7 @@
       Element element = backend.jsIndexableLength;
       bool isFixed = isFixedLength(actualReceiver.instructionType, compiler);
       HFieldGet result = new HFieldGet(
-          element, actualReceiver, backend.intType,
+          element, actualReceiver, backend.positiveIntType,
           isAssignable: !isFixed);
       return result;
     } else if (actualReceiver.isConstantMap()) {
@@ -830,12 +830,15 @@
                                  HInstruction indexArgument) {
     Compiler compiler = backend.compiler;
     HFieldGet length = new HFieldGet(
-        backend.jsIndexableLength, array, backend.intType,
+        backend.jsIndexableLength, array, backend.positiveIntType,
         isAssignable: !isFixedLength(array.instructionType, compiler));
     indexNode.block.addBefore(indexNode, length);
 
+    TypeMask type = indexArgument.isPositiveInteger(compiler)
+        ? indexArgument.instructionType
+        : backend.positiveIntType;
     HBoundsCheck check = new HBoundsCheck(
-        indexArgument, length, array, backend.intType);
+        indexArgument, length, array, type);
     indexNode.block.addBefore(indexNode, check);
     // If the index input to the bounds check was not known to be an integer
     // then we replace its uses with the bounds check, which is known to be an