dart-lang/sdk ? reverse-engineered prompt

Build me a polished developer toolkit for the Dart language, something that feels like the official SDK for writing, running, and compiling Dart apps on different platforms.

It should support a fast local workflow for app development, with good feedback while code changes, solid language features like strong typing and null safety, and clear tools for diagnosing problems. I want it to be able to target native apps, web apps, and server style programs, so the same codebase can be turned into machine code, JavaScript, or WebAssembly depending on what I need.

Please make it feel production ready, with clean documentation, helpful error messages, and a structure that would make sense for a real open source language project. If you need to look up current Dart docs online while building it, go ahead.