Isolate.spawn is broken in snapshots when run near a foreign .packages file

[nex3]

This breakage appears under the following conditions:

• The script contains a package: import.
• The script uses Isolate.spawn().
• The script is run from a snapshot.
• The directory containing the snapshot, or a parent of that directory, contains a .packages file that's different from the one used to create the snapshot.

To reproduce this, create a package with the following files:

// bin/bin.dart
import 'dart:isolate';

import 'package:app/app.dart';

main() async {
  await Isolate.spawn(entrypoint, null);
}

void entrypoint(_) {
}

// lib/app.dart

// This file can be empty.

# pubspec.yaml
name: app

Run:

$ pub get
$ dart --snapshot=bin.dart.snapshot bin/bin.dart
$ rm .packages
$ touch .packages
$ dart bin.dart.snapshot

You should see an error like the following:

Unhandled exception:
IsolateSpawnException: Unable to spawn isolate: Unhandled exception:
Load Error for "package:app/app.dart": No mapping for 'app' package when resolving 'package:app/app.dart'.
#0      _asyncLoadErrorCallback (dart:_builtin:155)
#1      _asyncLoadError (dart:_builtin:566)
#2      _loadPackage (dart:_builtin:605)
#3      _loadData (dart:_builtin:637)
#4      _loadDataAsync (dart:_builtin:657)
#5      _loadScriptCallback (dart:_builtin:153)
#6      _handleLoaderReply (dart:_builtin:370)
#7      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:148)
'file:///tmp/app/bin/bin.dart': error: line 3 pos 1: library handler failed
import 'package:app/app.dart';
^
#0      Isolate.spawn.<spawn_async_body> (dart:isolate-patch/isolate_patch.dart)
#1      _asyncErrorWrapperHelper.<anonymous closure> (dart:async-patch/async_patch.dart:34)
#2      _RootZone.runBinary (dart:async/zone.dart:1154)
#3      _Future._propagateToListeners.handleError (dart:async/future_impl.dart:579)
#4      _Future._propagateToListeners (dart:async/future_impl.dart:641)
#5      _Future._completeError (dart:async/future_impl.dart:432)
#6      _SyncCompleter._completeError (dart:async/future_impl.dart:56)
#7      _Completer.completeError (dart:async/future_impl.dart:27)
#8      Isolate._spawnCommon.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:413)
#9      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:148)

This is a regression. Running git bisect indicates that this error appeared as of 6d066c7. It's likely related to dart-lang/pub#1379.

[lrhn]

Doing Isolate.spawn should not read any files. It should run with exactly the same source and configuration as the isolate that calls spawn. If it rereads source files, that's no longer guaranteed, and if it's not running the same code, object transfer between the isolates may fail when the classes are no longer identical (or even there).
The new isolate should really be close to a "fork" of the original isolate - as it was just before running main.

[nex3]

@lrhn That's been an issue for a long time; see #6610.

[iposva-google]

Fixed: https://codereview.chromium.org/1591573004

Duplicate of #25430