dart2js cps: Share interceptors by default and propagate to use later.

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 15 matching lines @@
   print(l.length);
   for (int i  = 0; i < l.length; i++) {
     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, j;
+  var l = ["hest", ["h", "e", "s", "t"]], i = 0, x_, x, j;
   P.print(l.length);
   while (i < l.length) {
     if (i < 0 || i >= l.length)
       H.ioore(l, i);
-    x = l[i];
+    x_ = J.getInterceptor$as(x = l[i]);
     j = 0;
-    while (j < J.getInterceptor$as(x).get$length(x)) {
-      P.print(J.getInterceptor$as(x).$index(x, j));
+    while (j < x_.get$length(x)) {
+      P.print(x_.$index(x, j));
       j = j + 1;
     }
     i = i + 1;
   }
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }