Replace LinkedHashMap.* with Map.*, which _is_ Linked

[matanlurey]

Pre-work to make it easier to land #170435.

The default Map constructor is and always has been a LinkedHashMap:

https://api.dart.dev/dart-core/Map/Map.html

This change make its easier to relax the type annotation rules in a future PR.

/cc @eernstg

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[justinmc]

LGTM 👍

I guess after #170435 is merged then it won't be possible to add any more LinkedHashMaps into the codebase?