dart2js cps: Add a pass for eliminating bounds checks.

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 17 matching lines @@
     var x = l[i];
     for (int j = 0; j < x.length; j++) {
       print(x[j]);
     }
   }
 }""",
 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) {
-      if (j < 0 || j >= x.length)
+      if (j >= x.length)
         H.ioore(x, j);
       P.print(x[j]);
     }
   }
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }