From d7e7abbcc2dfac04861e257a35d5e6314f27e182 Mon Sep 17 00:00:00 2001
From: jensjoha <jensj@google.com>
Date: Mon, 15 Jan 2018 15:59:57 +0100
Subject: [PATCH] Train frontend_server snapshot (flutter/engine#4512)

Cuts 30% from hot-reload benchmarks with `--preview-dart-2`
---
 engine/src/flutter/frontend_server/BUILD.gn           |  3 ++-
 engine/src/flutter/frontend_server/lib/server.dart    | 11 +++++++++++
 .../src/flutter/frontend_server/test/server_test.dart |  6 ++++--
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/engine/src/flutter/frontend_server/BUILD.gn b/engine/src/flutter/frontend_server/BUILD.gn
index cebe1ec7b02..860ddc90003 100644
--- a/engine/src/flutter/frontend_server/BUILD.gn
+++ b/engine/src/flutter/frontend_server/BUILD.gn
@@ -7,7 +7,8 @@ import("//third_party/dart/utils/application_snapshot.gni")
 application_snapshot("frontend_server") {
   main_dart = "bin/starter.dart"
   dot_packages = rebase_path(".packages")
-  training_args = [ "--train" ]
+  flutter_patched_sdk = rebase_path("$root_out_dir/flutter_patched_sdk")
+  training_args = [ "--train", "--sdk-root=$flutter_patched_sdk" ]
 
   frontend_server_files = exec_script("//third_party/dart/tools/list_dart_files.py",
      [ "absolute", rebase_path("."), ], "list lines")
diff --git a/engine/src/flutter/frontend_server/lib/server.dart b/engine/src/flutter/frontend_server/lib/server.dart
index 3fce388e3d0..40906c23e29 100644
--- a/engine/src/flutter/frontend_server/lib/server.dart
+++ b/engine/src/flutter/frontend_server/lib/server.dart
@@ -261,6 +261,17 @@ 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();
+    await compiler.recompileDelta();
+    compiler.acceptLastDelta();
+    await compiler.recompileDelta();
+    compiler.acceptLastDelta();
     return 0;
   }
 
diff --git a/engine/src/flutter/frontend_server/test/server_test.dart b/engine/src/flutter/frontend_server/test/server_test.dart
index 1d2d215905b..513909e7319 100644
--- a/engine/src/flutter/frontend_server/test/server_test.dart
+++ b/engine/src/flutter/frontend_server/test/server_test.dart
@@ -26,8 +26,10 @@ class _MockedBinaryPrinter extends Mock implements BinaryPrinter {}
 
 Future<int> main() async {
   group('basic', () {
-    test('train completes', () async {
-      expect(await starter(<String>['--train']), equals(0));
+    final CompilerInterface compiler = new _MockedCompiler();
+
+    test('train with mocked compiler completes', () async {
+      expect(await starter(<String>['--train'], compiler: compiler), equals(0));
     });
   });