[dart2js] Deserialized JsScopeInfos don't get modified
[dart2js] Deserialized JsScopeInfos don't get modified

This means we can replace empty Maps and Sets with constant ones.
This reduces the heap for the link phase of a modular compile by 2.8%

Change-Id: If0b78d4f76f40e8b4628fabbf5fa2b9fa07db8be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177181
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Commit-Queue: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk