Object`fields` are not collapsed into groups of 100

Judging by the [implementation](https://github.com/flutter/devtools/blob/master/packages/devtools_app/lib/src/shared/object_tree.dart#L779), instance fields are not collapsed into groups of 100. This will cause issues for the record types implementation, which uses fields (see https://github.com/flutter/devtools/issues/5035).

We should make sure that objects with a large number of fields have those fields grouped into collapsible sections of 100.