Fix dataTransferItemList indexing

tools/dom/templates/html/impl/impl_DataTransferItemList.darttemplate

Index: tools/dom/templates/html/impl/impl_DataTransferItemList.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_DataTransferItemList.darttemplate b/tools/dom/templates/html/impl/impl_DataTransferItemList.darttemplate
index ab0442a556e1ac5714fdee33119096b177e86f9b..926ad9137a1da8e8adb97b78a0ff374cf4573625 100644
--- a/tools/dom/templates/html/impl/impl_DataTransferItemList.darttemplate
+++ b/tools/dom/templates/html/impl/impl_DataTransferItemList.darttemplate
@@ -15,7 +15,7 @@ $else
     // TODO(alanknight): I think that all the __getter__ generators should just
     // do property access, but that's major surgery. This one is a problem, so
     // just hard-code it for now.
-    return _blink.Blink_JsNative_DomException.getProperty(this, index);
+    return _blink.Blink_JsNative_DomException.getProperty(this, index.toString());
 $endif
   }