be pure and add dart

.drone.yml

@@ -5,6 +5,6 @@ name: default
 steps:
   - name: build
     commands:
-      - nix-shell --run "flutter build apk --split-per-abi"
+      - nix-shell --pure --run "flutter build apk --split-per-abi"
     environment:
       NIX_PATH: "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:/nix/var/nix/profiles/per-user/root/channels"

shell.nix

@@ -1,5 +1,5 @@
 { pkgs ? import <nixpkgs> {} }:
 
 pkgs.mkShell {
-  nativeBuildInputs = [ pkgs.flutter ];
+  nativeBuildInputs = with pkgs; [ flutter dart ];
 }