Add IDX (#2253)

PTAL @rodydavis


## Pre-launch Checklist

- [x] I read the [Effective Dart: Style] _recently_, and have followed
its advice.
- [x] I signed the [CLA].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Effective Dart: Style]:
https://dart.dev/guides/language/effective-dart/style
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md

---------

Co-authored-by: Rody Davis <[email protected]>

Author: domesticmouse

.idx/dev.nix

@@ -0,0 +1,28 @@
+# To learn more about how to use Nix to configure your environment
+# see: https://developers.google.com/idx/guides/customize-idx-env
+{ pkgs, ... }: {
+  # Which nixpkgs channel to use.
+  channel = "stable-24.05"; # or "unstable"
+
+  # Use https://search.nixos.org/packages to find packages
+  packages = [
+    pkgs.jdk17
+    pkgs.unzip
+  ];
+  
+  # Sets environment variables in the workspace
+  env = {};
+  idx = {
+    # Search for the extensions you want on https://open-vsx.org/ and use "publisher.id"
+    extensions = [
+      "Dart-Code.dart-code"
+      "Dart-Code.flutter"
+    ];
+    workspace = {
+      # Runs when a workspace is first created
+      onCreate = {
+        updateStable = "flutter channel stable && flutter upgrade && for dir in `find . -name pubspec.yaml -exec dirname {} \; | sort`; do pushd $dir ; flutter pub get; popd; done";
+      };
+    };
+  };
+}