fix: a more complete implementation of schema generation (#1550)

- parse out the default value from ctor param default
- exclude getter-only properties
- properly exclude properties not set via constructor from required set
- correctly include required nullable ctor params in required set

Fixes #1549

Author: kevmoo

json_serializable/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## 6.13.0-wip
 
+- Fix schema generation for many different types of fields. 
+  ([#1549](https://github.com/google/json_serializable.dart/issues/1549))
 - Support `JsonSerializable.dateTimeUtc` configuration option. 
   ([#1371](https://github.com/google/json_serializable.dart/issues/1371))
 - Remove `json_schema` from `dependencies` (it is only used in tests).

json_serializable/lib/src/field_helpers.dart

@@ -107,8 +107,4 @@ List<FieldElement> createSortedFieldSet(ClassElement element) {
   return fields.map((fs) => fs.field).toList(growable: false);
 }
 
-const _dartCoreObjectChecker = TypeChecker.typeNamed(
-  Object,
-  inPackage: 'core',
-  inSdk: true,
-);
+const _dartCoreObjectChecker = TypeChecker.fromUrl('dart:core#Object');