Write artifacts produced during frontend_server training into temp fo…

…lder. (flutter#4687)

* Write artifacts produced during frontend_server training into temp folder.

* Write artifacts produced during frontend_server training into temp folder.

* Fix File/String mixup

* Remove temp directory. Use full prefix for package

* Delete in finally {}

frontend_server/lib/server.dart

@@ -22,6 +22,7 @@ import 'package:kernel/binary/limited_ast_to_binary.dart';
 import 'package:kernel/kernel.dart' show Program, loadProgramFromBytes;
 import 'package:kernel/target/flutter.dart';
 import 'package:kernel/target/targets.dart';
+import 'package:path/path.dart' as path;
 import 'package:usage/uuid/uuid.dart';
 import 'package:vm/incremental_compiler.dart' show IncrementalCompiler;
 import 'package:vm/kernel_front_end.dart' show compileToKernel;
@@ -337,18 +338,30 @@ Future<int> starter(
 
   if (options['train']) {
     final String sdkRoot = options['sdk-root'];
-    options = _argParser.parse(<String>['--incremental', '--sdk-root=$sdkRoot']);
-    compiler ??= new _FrontendCompiler(output, printerFactory: binaryPrinterFactory);
-    await compiler.compile(Platform.script.toFilePath(), options, generator: generator);
-    compiler.acceptLastDelta();
-    await compiler.recompileDelta();
-    compiler.acceptLastDelta();
-    compiler.resetIncrementalCompiler();
-    await compiler.recompileDelta();
-    compiler.acceptLastDelta();
-    await compiler.recompileDelta();
-    compiler.acceptLastDelta();
-    return 0;
+    final Directory temp = Directory.systemTemp.createTempSync('train_frontend_server');
+    try {
+      final String outputTrainingDill = path.join(temp.path, 'app.dill');
+      options =
+          _argParser.parse(<String>['--incremental', '--sdk-root=$sdkRoot',
+          '--output-dill=$outputTrainingDill'
+          ]);
+      compiler ??=
+      new _FrontendCompiler(output, printerFactory: binaryPrinterFactory);
+
+      await compiler.compile(
+          Platform.script.toFilePath(), options, generator: generator);
+      compiler.acceptLastDelta();
+      await compiler.recompileDelta();
+      compiler.acceptLastDelta();
+      compiler.resetIncrementalCompiler();
+      await compiler.recompileDelta();
+      compiler.acceptLastDelta();
+      await compiler.recompileDelta();
+      compiler.acceptLastDelta();
+      return 0;
+    } finally {
+      temp.deleteSync(recursive: true);
+    }
   }
 
   compiler ??= new _FrontendCompiler(