External size is not accurate.

[polina-c]

this page in Flutter's documentation https://docs.flutter.dev/development/tools/devtools/memory#dart-size-vs-external-native-size needs an update, the engine does not provide an accurate external size value for some of things listed there

Suggestion: hide this column for normal users, and keep it just for vm developers

[bkonyi]

I'm fine with this for now as Flutter isn't reporting this information, but we do report external sizes for some dart:io classes. If another important embedder eventually decides to report external sizes, we might want to show these values by default.