Give each nested test run its own build directory

test/util/run_test_in_process.dart

@@ -1,8 +1,10 @@
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:flutter/foundation.dart';
 import 'package:spot/src/flutter/flutter_sdk.dart';
+import 'package:spot/src/utils/invoker.dart';
 import 'package:test/test.dart';
 import 'package:test_process/test_process.dart';
 
@@ -38,6 +40,7 @@ Future<String?> runTestInProcessAndCaptureOutPut({
     arguments,
     environment: {
       'CI': 'true',
+      ..._ownBuildDirectory(),
     },
   );
   final stdoutBuffer = StringBuffer();
@@ -81,6 +84,86 @@ Future<String?> runTestInProcessAndCaptureOutPut({
   return stdout;
 }
 
+/// Environment that sends the nested `flutter test` to a build directory of its
+/// own instead of the one this repository is being tested in.
+///
+/// The nested process inherits the working directory, so without this every run
+/// writes into the same `build/`. `flutter test` empties
+/// `build/native_assets/<os>/` on startup, writes the `native_assets.json` it
+/// copies into the test assets into that same directory, and copies it only
+/// after building the asset bundle. A run that starts while another one is
+/// between those two steps deletes the file that run is about to copy:
+///
+/// ```text
+/// Flutter failed to copy file from ".../build/native_assets/linux/native_assets.json"
+/// to ".../build/unit_test_assets/NativeAssetsManifest.json".
+/// The file or directory could not be found.
+/// ```
+///
+/// The build directory is only settable through Flutter's config file, so the
+/// nested process gets a config file of its own. Every test file gets one build
+/// directory, not every run: runs of one file are sequential and cannot collide,
+/// and sharing keeps their compilation cache warm.
+Map<String, String> _ownBuildDirectory() {
+  final configDir = Directory.systemTemp.createTempSync('spot_flutter_config');
+  addTearDown(() {
+    _deleteTempDir(configDir);
+  });
+
+  final settings = jsonEncode({'build-dir': _buildDirectoryOfTestFile()});
+  // Flutter looks for `$HOME/.flutter_settings` first and falls back to
+  // `$XDG_CONFIG_HOME/settings`, on Windows it reads `$APPDATA/.flutter_settings`.
+  // Both files exist here and all three variables point at them, so the config
+  // the nested run finds is this one whichever rule it follows. Pointing only
+  // `XDG_CONFIG_HOME` here would leave machines that still carry the long
+  // deprecated `$HOME/.flutter_settings` with the shared build directory.
+  File('${configDir.path}/settings').writeAsStringSync(settings);
+  File('${configDir.path}/.flutter_settings').writeAsStringSync(settings);
+
+  final pubCache = _pubCache();
+  return {
+    'HOME': configDir.path,
+    'XDG_CONFIG_HOME': configDir.path,
+    'APPDATA': configDir.path,
+    if (pubCache != null) 'PUB_CACHE': pubCache,
+  };
+}
+
+/// The package cache this process resolved its own dependencies from.
+///
+/// It is named explicitly because the nested run looks for it in the home
+/// directory it just lost, and would download everything again into the
+/// throwaway one.
+String? _pubCache() {
+  final configured = Platform.environment['PUB_CACHE'];
+  if (configured != null) {
+    return configured;
+  }
+  final home = Platform.environment['HOME'];
+  if (home == null) {
+    return null;
+  }
+  return '$home/.pub-cache';
+}
+
+/// A directory under `build/`, named after the test file that is running.
+///
+/// The path has to be relative, Flutter rejects an absolute one.
+String _buildDirectoryOfTestFile() {
+  final path = getLiveTest()?.suite.path;
+  if (path == null) {
+    // Only null outside a running test, which is not where nested runs start.
+    return 'build/nested_test/unknown';
+  }
+  final current = Directory.current.path;
+  final relative =
+      path.startsWith(current) ? path.substring(current.length) : path;
+  final name = relative
+      .replaceAll(RegExp('[^A-Za-z0-9]+'), '_')
+      .replaceFirst(RegExp('^_'), '');
+  return 'build/nested_test/$name';
+}
+
 Future<File> _createTempTestFile(String content) async {
   final tempDir = Directory.systemTemp.createTempSync();
   final tempTestFile = File('${tempDir.path}/temp_test.dart');