Place hot reload artifacts in a temp directory (#40171)

diff --git a/packages/flutter_tools/lib/src/resident_runner.dart b/packages/flutter_tools/lib/src/resident_runner.dart
index 770253d..b373751 100644
--- a/packages/flutter_tools/lib/src/resident_runner.dart
+++ b/packages/flutter_tools/lib/src/resident_runner.dart

@@ -536,34 +536,33 @@
     this.ipv6,
     this.stayResident = true,
     this.hotMode = true,
-    this.dillOutputPath,
+    String dillOutputPath,
   }) : mainPath = findMainDartFile(target),
        projectRootPath = projectRootPath ?? fs.currentDirectory.path,
        packagesFilePath = packagesFilePath ?? fs.path.absolute(PackageMap.globalPackagesPath),
+       _dillOutputPath = dillOutputPath,
+       artifactDirectory = dillOutputPath == null
+          ? fs.systemTempDirectory.createTempSync('_fluttter_tool')
+          : fs.file(dillOutputPath).parent,
        assetBundle = AssetBundleFactory.instance.createBundle() {
-    // TODO(jonahwilliams): this is transitionary logic to allow us to support
-    // platforms that are not yet using flutter assemble. In the "new world",
-    // builds are isolated based on a number of factors. Thus, we cannot assume
-    // that a debug build will create the expected `build/app.dill` file. For
-    // now, I'm working around this by just creating it if it is missing here.
-    // In the future, once build & run are more strongly separated, the build
-    // environment will be plumbed through so that it all comes from a single
-    // source of truth, the [Environment].
-    final File dillOutput = fs.file(dillOutputPath ?? fs.path.join('build', 'app.dill'));
-    if (!dillOutput.existsSync()) {
-      dillOutput.createSync(recursive: true);
+    if (!artifactDirectory.existsSync()) {
+      artifactDirectory.createSync(recursive: true);
     }
   }
 
   @protected
   @visibleForTesting
   final List<FlutterDevice> flutterDevices;
+
   final String target;
   final DebuggingOptions debuggingOptions;
   final bool stayResident;
   final bool ipv6;
+  final String _dillOutputPath;
+  /// The parent location of the incremental artifacts.
+  @visibleForTesting
+  final Directory artifactDirectory;
   final Completer<int> _finished = Completer<int>();
-  final String dillOutputPath;
   final String packagesFilePath;
   final String projectRootPath;
   final String mainPath;
@@ -571,6 +570,8 @@
 
   bool _exited = false;
   bool hotMode ;
+
+  String get dillOutputPath => _dillOutputPath ?? fs.path.join(artifactDirectory.path, 'app.dill');
   String getReloadPath({ bool fullRestart }) => mainPath + (fullRestart ? '' : '.incremental') + '.dill';
 
   bool get debuggingEnabled => debuggingOptions.debuggingEnabled;
@@ -716,6 +717,7 @@
   /// Throws an [AssertionError] if [Devce.supportsScreenshot] is not true.
   Future<void> screenshot(FlutterDevice device) async {
     assert(device.device.supportsScreenshot);
+
     final Status status = logger.startProgress('Taking screenshot for ${device.device.name}...', timeout: timeoutConfiguration.fastOperation);
     final File outputFile = getUniqueFile(fs.currentDirectory, 'flutter', 'png');
     try {
@@ -850,7 +852,13 @@
     return exitCode;
   }
 
-  Future<void> preExit() async { }
+  @mustCallSuper
+  Future<void> preExit() async {
+    // If _dillOutputPath is null, we created a temporary directory for the dill.
+    if (_dillOutputPath == null && artifactDirectory.existsSync()) {
+      artifactDirectory.deleteSync(recursive: true);
+    }
+  }
 
   Future<void> exitApp() async {
     final List<Future<void>> futures = <Future<void>>[];

diff --git a/packages/flutter_tools/lib/src/run_cold.dart b/packages/flutter_tools/lib/src/run_cold.dart
index b621c78..ddfde2e 100644
--- a/packages/flutter_tools/lib/src/run_cold.dart
+++ b/packages/flutter_tools/lib/src/run_cold.dart

@@ -206,5 +206,6 @@
       if (device.vmServices == null || device.vmServices.isEmpty)
         await device.device.stopApp(device.package);
     }
+    await super.preExit();
   }
 }

diff --git a/packages/flutter_tools/lib/src/run_hot.dart b/packages/flutter_tools/lib/src/run_hot.dart
index 3be3253..f9b590a 100644
--- a/packages/flutter_tools/lib/src/run_hot.dart
+++ b/packages/flutter_tools/lib/src/run_hot.dart

@@ -1010,6 +1010,7 @@
   Future<void> preExit() async {
     await _cleanupDevFS();
     await hotRunnerConfig.runPreShutdownOperations();
+    await super.preExit();
   }
 
   @override