[native_assets_cli] Separate out json_syntax_generator into separate package (#2101)

Bug: #1826

This refactors `package:hook`s `tool/generate_syntax.dart`.

* Moved into its own package `package:json_syntax_generator`.
* Documented the feature set: optional/required fields, subclassing, tagged unions, and enums.
* Refactored from a single-pass code generator, to a two step pipeline. (1) analyze the JSON schema and extra into a data structure, (2) generate code from the data structure.
* Sprinkled comments about the "why" on the most important classes and fields.

This PR is intended to be a NOP w.r.t. the syntax that's generated. (Some whitespace and `toString` changes.)

Having this as a separate package will enable us to reuse it for the syntax in different places. For now, we're not intending to publish it, instead we can use it as a git dependency in the dev dependencies (or we publish it unlisted). @mosuem Please let me know if it is possible for you to generate the syntax for the `FontAsset`s.

Open issue:

* The generated code requires a `utils/json.dart`. We should make the generated code be standalone. I'll do that in a follow up PR.

Author: dcharkes

pkgs/hook/pubspec.yaml

@@ -10,5 +10,7 @@ environment:
 dev_dependencies:
   dart_flutter_team_lints: ^2.1.1
   json_schema: ^5.2.0
+  json_syntax_generator:
+    path: ../json_syntax_generator/
   path: ^1.9.1
   test: ^1.25.15

pkgs/hook/test/schema/helpers.dart

@@ -96,6 +96,16 @@ AllSchemas loadSchemas(List<Uri> directories) {
     );
     allSchemas[entry.key] = schema;
   }
+
+  final allSchemasInverted = allSchemas.map(
+    (key, value) => MapEntry(value, key),
+  );
+  if (allSchemas.length != allSchemasInverted.length) {
+    throw StateError(
+      'Some schemas are not unique, try adding a unique title field.',
+    );
+  }
+
   return allSchemas;
 }
 

pkgs/hook/tool/generate_schemas.dart

@@ -6,7 +6,6 @@ import 'dart:convert';
 import 'dart:io';
 
 import '../test/schema/helpers.dart';
-import 'generate_syntax.dart';
 import 'normalize.dart';
 
 void main() {
@@ -184,3 +183,10 @@ void generateEntryPoints() {
 
 String definitionName(Hook hook, InputOrOutput inputOrOutput) =>
     '${ucFirst(hook.name)}${ucFirst(inputOrOutput.name)}';
+
+String ucFirst(String str) {
+  if (str.isEmpty) {
+    return '';
+  }
+  return str[0].toUpperCase() + str.substring(1);
+}