feat: Node.js server target — compile Dart APIs to Node.js

[flutterjs-dev]

Adds a full Node.js server compilation target to FlutterJS, enabling developers to write Dart REST APIs and compile them directly to Node.js JavaScript. Includes the flutterjs_server package, a working dart_api example, and all the compiler fixes needed to make server-side code generation reliable.

🔗 Related Issue

Fixes # (none — new feature)

🏗️ Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🐛 Bug fix (non-breaking change which fixes an issue)

---

What's New

flutterjs_server Package

A new Dart package + JS runtime for writing HTTP servers in Dart:

• FlutterjsServer — HTTP server with port configuration
• Router — route registration (add, mount)
• Request / Response — request/response model with ok, created, notFound, badRequest, noContent factories
• cors() / logger() / bearerAuth() — built-in middleware

dart_api Example

A complete REST API demo compiled from Dart → Node.js:

• GET / — welcome message
• GET /health — health check
• GET /users / GET /users/:id — list and get users
• POST /users — create user
• DELETE /users/:id — delete user

Compiler Fixes

Import generation:

• CascadeExpressionIR now scanned in ImportAnalyzer and FileCodeGen — cascade chains like server..use(cors())..use(logger()) now correctly detect and import cors/logger
• GlobalSymbolTable wired to ModelToJSPipeline after package manifests load (was empty before, causing missing imports)
• PackageRegistry.buildGlobalSymbolTable() maps symbols to full package: URIs for correct path resolution

Code generation:

• Dart == null / != null now generates loose JS == null to catch both null and undefined
• as Map<String, dynamic> cast uses typeof x === 'object' && !Array.isArray(x) instead of broken instanceof Map
• Removed hardcoded identifier→this.identifier hacks that broke local variables
• Added EnumDecl IR node and EnumCodeGenerator for proper enum → JS object output
• Added ImportExportTracker and ImportExportModel for accurate cross-file import analysis

Node target pipeline:

• PackageRegistry: skips web-only @flutterjs packages (material, widgets, vdom, seo, rendering, etc.) when --target node to prevent symbol table pollution
• RuntimePackageManager: when linking @flutterjs/X, also links as flutterjs_X in root node_modules so generated import 'flutterjs_server' resolves
• RunCommand: auto-creates node_modules junctions before starting the node server — no flutterjs get required for basic use

---

📋 Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• The dart_api example compiles and all endpoints pass smoke tests

---

🧪 How to Test

# 1. From the repo root, run the dart_api example
cd examples/dart_api
dart run ../../bin/flutterjs.dart run --to-js --target node --serve --devtools-no-open

# 2. Test the endpoints
curl http://localhost:3000/
curl http://localhost:3000/health
curl http://localhost:3000/users
curl http://localhost:3000/users/1
curl http://localhost:3000/users/99          # → 404
curl -X POST http://localhost:3000/users \
     -H "Content-Type: application/json" \
     -d '{"name":"Dave","email":"dave@example.com"}'
curl -X DELETE http://localhost:3000/users/2

Expected: All endpoints return correct JSON with appropriate HTTP status codes (200, 201, 204, 404).