dart2js cps: Add more constraint rules to bounds-check elimination.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 906ed6f531333462d5c04577672944bcb82a697b..c89173ad0cf60ca506e49032c8a664a98529417c 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -1233,10 +1233,12 @@ class TransformingVisitor extends DeepRecursiveVisitor {
       return cps;
     }
     Continuation fail = cps.letCont();
-    Primitive isTooSmall = cps.applyBuiltin(
-        BuiltinOperator.NumLt,
-        <Primitive>[index, cps.makeZero()]);
-    cps.ifTruthy(isTooSmall).invokeContinuation(fail);
+    if (!typeSystem.isDefinitelyNonNegativeInt(index.type)) {
+      Primitive isTooSmall = cps.applyBuiltin(
+          BuiltinOperator.NumLt,
+          <Primitive>[index, cps.makeZero()]);
+      cps.ifTruthy(isTooSmall).invokeContinuation(fail);
+    }
     Primitive isTooLarge = cps.applyBuiltin(
         BuiltinOperator.NumGe,
         <Primitive>[index, cps.letPrim(new GetLength(list))]);