[native_assets_cli] Split up syntax generator

[dcharkes]

Bug: #1826

More refactorings to package:json_syntax_generator.

• Split up generator in multiple files.
• Move more logic to the helper functions.
  • setOrRemove to remove branching in setters for nullable values.
  • optionalListParsed to immediately parse list elements, to remove loops in getters.
• Made all getters and setters use $dartType instead of spelling out types. This simplified generation of getters/setters for simplified types to a simple 10 lines. (Yay for a nice DartType abstraction!)
• Removed the if (required) branching and instead made the relevant parts be variables.

[github-actions]

PR Health

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License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 86.642% (+0.06%) from 86.587%
when pulling 90005a5 on refactor-generator-3
into 329b975 on main.

[HosseinYousefi]

Great minds think alike: https://github.com/dart-lang/native/blob/main/pkgs/jnigen/lib/src/bindings/dart_generator.dart#L61-L66

[dcharkes]

Haha nice! 😄