Remove reference to token.previous

Change-Id: Ia1ae04e03b868dfb482d4853fac1c288331a6085
Reviewed-on: https://dart-review.googlesource.com/55942
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Dan Rubel <danrubel@google.com>
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
index e37be19..1291f78 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
@@ -22,7 +22,7 @@
   if (node is ArgumentList) {
     if (request.target.entity == node.rightParenthesis) {
       // Parser ignores trailing commas
-      if (node.rightParenthesis.previous?.lexeme == ',') {
+      if (node.findPrevious(node.rightParenthesis)?.lexeme == ',') {
         return node.arguments.length + 1;
       }
     }