Add an option to turn off default values stripping by default.

Author: BirjuVachhani

build.yaml

@@ -8,4 +8,5 @@ targets:
           explicit_to_json: true
           # include_if_null should be false to avoid sending null properties to Firebase.
           include_if_null: false
-          any_map: true # The squadron library for web requires dynamic maps.
+          any_map: true # The squadron library for web requires dynamic maps.
+          serialize_default_values: false