chore(mason): loosen pkg:collection constraint for Flutter compat (#1411)

Author: felangel

packages/mason/pubspec.yaml

@@ -15,7 +15,7 @@ environment:
 dependencies:
   archive: ^3.6.1
   checked_yaml: ^2.0.3
-  collection: ^1.19.0
+  collection: ^1.18.0
   convert: ^3.1.1
   crypto: ^3.0.5
   http: ^1.2.2