Fix broken file level comment

Change-Id: I57199a33f6c3e450195f96d3bfdc8091f3f47456
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
index 08414ae..8ac47a2 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
@@ -561,7 +561,13 @@
       );
       return;
     }
-    this._createComment(el, lineNum);
+
+    // TODO(TS): existing logic always pass undefined lineNum
+    // for file level comment, the drafts API will reject the
+    // request if file level draft contains the `line: 'FILE'` field
+    // probably should do this inside of the _createComment, this
+    // is just to keep existing behavior.
+    this._createComment(el, lineNum === FILE ? undefined : lineNum);
   }
 
   createRangeComment() {
@@ -644,7 +650,7 @@
 
   _createComment(
     lineEl: Element,
-    lineNum: LineNumber,
+    lineNum?: LineNumber,
     side?: Side,
     range?: CommentRange
   ) {