Reapply flutter_assets (#13567)

* Revert "Revert engine (#13547)"

This reverts commit ee65db11d4618620ddc286f4618b707239205136.

* Keep flaky mark
diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index 154a2ec..5f3aef2 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart
@@ -202,7 +202,7 @@
 
 /// Returns the asset build output directory.
 String getAssetBuildDirectory() {
-  return fs.path.join(getBuildDirectory(), 'flx');
+  return fs.path.join(getBuildDirectory(), 'flutter_assets');
 }
 
 /// Returns the iOS build output directory.