for-of should throw if result object is not an object

This is done using desugaring. Before this we had:

  result = iterator.next()

with this we instead do:

  !%_IsSpecObject(result = iterator.next()) &&
      %ThrowIteratorResultNotAnObject(result)

BUG=v8:3916
LOG=N

Review URL: https://codereview.chromium.org/929733003

Cr-Commit-Position: refs/heads/master@{#26806}

src/runtime/runtime-internal.cc

@@ -65,6 +65,16 @@ RUNTIME_FUNCTION(Runtime_ThrowIteratorResultNotAnObject) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_ThrowIteratorResultNotAnObject) {
+  HandleScope scope(isolate);
+  DCHECK(args.length() == 1);
+  CONVERT_ARG_HANDLE_CHECKED(Object, value, 0);
+  THROW_NEW_ERROR_RETURN_FAILURE(
+      isolate,
+      NewTypeError("iterator_result_not_an_object", HandleVector(&value, 1)));
+}
+
+
 RUNTIME_FUNCTION(Runtime_PromiseRejectEvent) {
   DCHECK(args.length() == 3);
   HandleScope scope(isolate);