Mark form_data_test as failing in dart2js.

tests/html/html.status

Index: tests/html/html.status
diff --git a/tests/html/html.status b/tests/html/html.status
index 775f19b1177df2ec4d1571940b7908426b8ba9d7..f1b146881ea58bfd7c32391631cb1e961c912239 100644
--- a/tests/html/html.status
+++ b/tests/html/html.status
@@ -211,6 +211,7 @@ request_animation_frame_test: Skip # Async test hangs.
 [ $compiler == dart2js && ($runtime == drt || $runtime == chrome || $runtime == ff || $runtime == safari) ]
 isolates_test: Skip   # Timeout because leg does not support web workers.
 typed_arrays_1_test: Fail # dart2js doesn't support is checks with generic types.
+form_data_test: Fail # Wrong JavaScript type name for FormData, issue 5681.
 
 [ $compiler == dart2js && $runtime == ff && $system == linux]
 indexeddb_3_test: Fail  # Caught Error: DATA_ERR: DOM IDBDatabase Exception 5 Error: The data provided does not meet the requirements of the function.