Place existing dill into hot reload temp directory to boost initialization time (#40366)

packages/flutter_tools/lib/src/resident_runner.dart

@@ -530,6 +530,16 @@ abstract class ResidentRunner {
     if (!artifactDirectory.existsSync()) {
       artifactDirectory.createSync(recursive: true);
     }
+    // TODO(jonahwilliams): this is a temporary work around to regain some of
+    // the initialize from dill performance. Longer term, we should have a
+    // better way to determine where the appropriate dill file is, as this
+    // doesn't work for Android or macOS builds.}
+    if (dillOutputPath == null) {
+      final File existingDill = fs.file(fs.path.join('build', 'app.dill'));
+      if (existingDill.existsSync()) {
+        existingDill.copySync(fs.path.join(artifactDirectory.path, 'app.dill'));
+      }
+    }
   }
 
   @protected

packages/flutter_tools/test/general.shard/resident_runner_test.dart

@@ -35,6 +35,9 @@ void main() {
 
   setUp(() {
     testbed = Testbed(setup: () {
+      fs.file(fs.path.join('build', 'app.dill'))
+        ..createSync(recursive: true)
+        ..writeAsStringSync('ABC');
       residentRunner = HotRunner(
         <FlutterDevice>[
           mockFlutterDevice,
@@ -193,6 +196,10 @@ void main() {
     Usage: () => MockUsage(),
   }));
 
+  test('ResidentRunner copies dill file from build output into temp directory', () => testbed.run(() async {
+    expect(residentRunner.artifactDirectory.childFile('app.dill').readAsStringSync(), 'ABC');
+  }));
+
   test('ResidentRunner can send target platform to analytics from hot reload', () => testbed.run(() async {
     when(mockDevice.sdkNameAndVersion).thenAnswer((Invocation invocation) async {
       return 'Example';