Don't take iterable path in ArrayFrom if items[@@iterator] is null

src/harmony-array.js

Index: src/harmony-array.js
diff --git a/src/harmony-array.js b/src/harmony-array.js
index 762f746e56d923f1330ac177fa41403a331c18c8..5a4010ffe3d974bb9fb2806ae2b188a66f74ee83 100644
--- a/src/harmony-array.js
+++ b/src/harmony-array.js
@@ -142,7 +142,7 @@ function ArrayFrom(arrayLike, mapfn, receiver) {
     }
   }
 
-  var iterable = ToIterable(items);
+  var iterable = GetMethod(items, symbolIterator);

[arv (Not doing code reviews), 2015/01/21 15:05:18]

We now call items.[[Get]](symbolIterator) twice. The spec mandates that it is
only called once. The spec makes me sad that we cannot use for-of loop here.

[caitp (gmail), 2015/01/21 17:21:49]

hmm, well it's trivial to replace the for...of with a more explicit loop, but
that is a bit sad.

   var k;
   var result;
   var mappedValue;