Fix quill_delta next op

lib/models/quill_delta.dart

@@ -639,7 +639,7 @@ class DeltaIterator {
   ///
   /// Optional [length] specifies maximum length of operation to return. Note
   /// that actual length of returned operation may be less than specified value.
-  Operation next([num length = double.infinity]) {
+  Operation next([int length = 4294967296]) {
     assert(length != null);
 
     if (_modificationCount != delta._modificationCount) {