[dart] Fix enum defaults, Object arrays, and nullable required assertions in native serialization

[EddieRydell]

Summary

Fixes three bugs in the Dart native serialization codegen:

• Enum default values emit string literals instead of enum constructors — fromJson fallback generated ?? 'value_one' instead of ?? const EnumType._('value_one'), causing a type mismatch
• Object.listFromJson() doesn't exist — Object was not in languageSpecificPrimitives, so Object-typed arrays generated invalid Object.listFromJson() calls instead of cast<Object>()
• Required+nullable fields incorrectly assert non-null — the requiredKeys.forEach assertion block asserted json[key] != null for all required fields, including nullable ones

Fixes #13459, fixes #20152, fixes #20296

Changes

File	Change
AbstractDartCodegen.java	Add "Object" to languageSpecificPrimitives (consistent with DartDio)
native_class.mustache	Wrap enum default with const {enumName}._() constructor
native_class.mustache	Replace requiredKeys.forEach with per-property assertions respecting isNullable
DartClientCodegenTest.java	Add integration tests for all three fixes

Test plan

• New integration tests generate Dart code from a test spec and assert correct output
• All 109 Dart tests pass (native + DartDio)
• Regenerated samples

---

Summary by cubic

Fixes three Dart native serialization bugs: enum default deserialization, Object arrays, and required+nullable field assertions. Also updates docs to list Object as a language primitive in Dart and Dart-Dio.

• Bug Fixes

  • Enum defaults in fromJson now use const Enum._('value') instead of a string literal.
  • Object-typed arrays no longer call Object.listFromJson(); Object is treated as a primitive and uses cast().
  • Required nullable fields assert presence only; non-null checks apply only to non-nullable required fields.

  • Migration

    • Regenerate Dart native clients to pick up the fixes. No manual changes needed.

  ^{Written for commit 1a9997f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 60 files

[wing328]

thanks for the PR which has been merged

[wing328]

FYI @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)