dart2js cps: Add path-sensitive types by inserting refinement IR nodes.

tests/compiler/dart2js/js_backend_cps_ir_interceptors_test.dart

 // Copyright (c) 2014, 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.
 
 // Tests of interceptors.
 
 library interceptors_tests;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 20 matching lines @@
     }
   }
 }""",
 r"""
 function() {
   var l = ["hest", ["h", "e", "s", "t"]], i = 0, x_, x, j;
   for (P.print(l.length); i < l.length; i = i + 1) {
     if (i < 0 || i >= l.length)
       H.ioore(l, i);
     x_ = J.getInterceptor$as(x = l[i]);
-    for (j = 0; j < x_.get$length(x); j = j + 1)
-      P.print(x_.$index(x, j));
+    for (j = 0; j < x_.get$length(x); j = j + 1) {
+      if (j < 0 || j >= x.length)
+        H.ioore(x, j);
+      P.print(x[j]);
+    }
   }
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }