Force LANG=en_US.UTF-8 in test runner (#82308)

dev/bots/test/bot_test.dart

@@ -1,13 +0,0 @@
-// Copyright 2014 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'dart:io';
-
-import 'common.dart';
-
-void main() {
-  test('BOT variable is set on bots', () {
-    expect(Platform.environment['BOT'], 'true');
-  }, skip: true);
-}

dev/devicelab/lib/framework/utils.dart

@@ -279,10 +279,10 @@ Future<Process> startProcess(
   assert(isBot != null);
   final String command = '$executable ${arguments?.join(" ") ?? ""}';
   final String finalWorkingDirectory = workingDirectory ?? cwd;
-  print('\nExecuting: $command in $finalWorkingDirectory'
-      + (environment != null ? ' with environment $environment' : ''));
   final Map<String, String> newEnvironment = Map<String, String>.from(environment ?? <String, String>{});
   newEnvironment['BOT'] = isBot ? 'true' : 'false';
+  newEnvironment['LANG'] = 'en_US.UTF-8';
+  print('\nExecuting: $command in $finalWorkingDirectory with environment $newEnvironment');
   final Process process = await _processManager.start(
     <String>[executable, ...arguments],
     environment: newEnvironment,

dev/devicelab/test/tasks/build_test_task_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
 import 'package:flutter_devicelab/framework/runner.dart';
 import 'package:flutter_devicelab/framework/task_result.dart';
 
@@ -43,6 +45,30 @@ void main() {
     expect(result.data['benchmark'], 'data');
   });
 
+  test('sets environment', () async {
+    final StringBuffer capturedPrintLines = StringBuffer();
+    await runZoned<Future<void>>(
+      () async {
+        await runTask(
+          'smoke_test_build_test',
+          taskArgs: <String>['--test'],
+          deviceId: 'FAKE_SUCCESS',
+          isolateParams: isolateParams,
+        );
+      },
+      zoneSpecification: ZoneSpecification(
+        // Intercept printing from the task.
+        print: (Zone self, ZoneDelegate parent, Zone zone, String line) async {
+          capturedPrintLines.writeln(line);
+        },
+      ),
+    );
+    final String capturedPrint = capturedPrintLines.toString();
+    expect(capturedPrint,
+        contains('with environment {FLUTTER_DEVICELAB_DEVICEID: FAKE_SUCCESS, BOT: true, LANG: en_US.UTF-8}'));
+    expect(capturedPrint, contains('exit code: 0'));
+  });
+
   test('throws exception when build and test arg are given', () async {
     final TaskResult result = await runTask(
       'smoke_test_build_test',