flutter: use engine dart when available

pkgs/development/compilers/flutter/engine/dart.nix

@@ -0,0 +1,13 @@
+{ engine, runCommand }:
+runCommand "flutter-engine-${engine.version}-dart" {
+  version = engine.dartSdkVersion;
+
+  inherit engine;
+  inherit (engine) outName;
+
+  meta = engine.meta // {
+    description = "Dart SDK compiled from the Flutter Engine";
+  };
+} ''
+  ln -s ${engine}/out/$outName/dart-sdk $out
+''

pkgs/development/compilers/flutter/engine/default.nix

@@ -44,7 +44,9 @@ stdenv.mkDerivation (
       dartSdkVersion
       isOptimized
       runtimeMode
-      outName;
+      outName
+      dart
+      ;
     inherit altRuntimeMode;
 
     dontUnpack = true;

pkgs/development/compilers/flutter/engine/package.nix

@@ -60,7 +60,7 @@ let
 
   outName = "host_${runtimeMode}${lib.optionalString (!isOptimized) "_unopt --unoptimized"}";
 in
-stdenv.mkDerivation {
+stdenv.mkDerivation (finalAttrs: {
   pname = "flutter-engine-${runtimeMode}${lib.optionalString (!isOptimized) "-unopt"}";
   inherit
     version
@@ -307,6 +307,10 @@ stdenv.mkDerivation {
     runHook postInstall
   '';
 
+  passthru = {
+    dart = callPackage ./dart.nix { engine = finalAttrs.finalPackage; };
+  };
+
   meta = {
     # Very broken on Darwin
     broken = stdenv.isDarwin;
@@ -321,4 +325,4 @@ stdenv.mkDerivation {
       "aarch64-darwin"
     ];
   };
-}
+})

pkgs/development/compilers/flutter/flutter.nix

@@ -19,19 +19,13 @@
 , git
 , which
 , jq
-, flutterTools ? callPackage ./flutter-tools.nix {
-    inherit dart version;
-    flutterSrc = src;
-    inherit patches;
-    inherit pubspecLock;
-    systemPlatform = stdenv.hostPlatform.system;
-  }
+, flutterTools ? null
 }@args:
 
 let
   engine = if args.useNixpkgsEngine or false then
     callPackage ./engine/default.nix {
-      dartSdkVersion = dart.version;
+      dartSdkVersion = args.dart.version;
       flutterVersion = version;
       version = engineVersion;
       hashes = engineHashes;
@@ -40,6 +34,17 @@ let
       runtimeModes = engineRuntimeModes;
     } else null;
 
+  dart = if args.useNixpkgsEngine or false then
+    engine.dart else args.dart;
+
+  flutterTools = args.flutterTools or (callPackage ./flutter-tools.nix {
+    inherit dart version;
+    flutterSrc = src;
+    inherit patches;
+    inherit pubspecLock;
+    systemPlatform = stdenv.hostPlatform.system;
+  });
+
   unwrapped =
     stdenv.mkDerivation {
       name = "flutter-${version}-unwrapped";