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

test/mjsunit/elements-transition.js

Index: test/mjsunit/elements-transition.js
diff --git a/test/mjsunit/elements-transition.js b/test/mjsunit/elements-transition.js
index 7b907256dfc6d4fbabf0980df725e70d72b6c0eb..5f6cc4fa3763a0edbe48129fe9fec80f7cb2d22d 100644
--- a/test/mjsunit/elements-transition.js
+++ b/test/mjsunit/elements-transition.js
@@ -89,6 +89,19 @@ if (support_smi_only_arrays) {
   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.");
 }