Handle COW-arrays correctly when converting smi->double fast elements.

test/mjsunit/elements-transition.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 71 matching lines @@
 
   test(false, false, function(a,i,v){ a[i] = v; }, 20);
   test(true,  false, function(a,i,v){ a[i] = v; }, 20);
   test(false, true,  function(a,i,v){ a[i] = v; }, 20);
   test(true,  true,  function(a,i,v){ a[i] = v; }, 20);
 
   test(false, false, function(a,i,v){ a[i] = v; }, 10000);
   test(true,  false, function(a,i,v){ a[i] = v; }, 10000);
   test(false, true,  function(a,i,v){ a[i] = v; }, 10000);
   test(true,  true,  function(a,i,v){ a[i] = v; }, 10000);
+
+  // Check COW arrays
+  function get_cow() { return [1, 2, 3]; }
+
+  function transition(x) { x[0] = 1.5; }
+
+  var ignore = get_cow();
+  transition(ignore);  // Handled by runtime.
+  var a = get_cow();
+  var b = get_cow();
+  transition(a);  // Handled by IC.
+  assertEquals(1.5, a[0]);
+  assertEquals(1, b[0]);
 } else {
   print("Test skipped because smi only arrays are not supported.");
 }