Fix dataTransferItemList indexing

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index e007fc345d02570218cf85f366cd76564ce57ed4..b3be18227b432e69924825b780687b1c80d77f45 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -9381,7 +9381,7 @@ class DataTransferItemList extends DartHtmlDomObject {
     // 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());
   }
 
 }