Fix flush issue

testing/scenario_app/bin/android_integration_tests.dart

@@ -91,6 +91,8 @@ void main(List<String> args) async {
   });
 
   late Process logcatProcess;
+  late Future<int> logcatProcessExitCode;
+
   final IOSink logcat = File(logcatPath).openWrite();
   try {
     await step('Creating screenshot directory...', () async {
@@ -103,7 +105,7 @@ void main(List<String> args) async {
         panic(<String>['could not clear logs']);
       }
       logcatProcess = await pm.start(<String>[adb.path, 'logcat', '-T', '1']);
-      unawaited(pipeProcessStreams(logcatProcess, out: logcat));
+      logcatProcessExitCode = pipeProcessStreams(logcatProcess, out: logcat);
     });
 
     await step('Configuring emulator...', () async {
@@ -209,6 +211,7 @@ void main(List<String> args) async {
     await step('Killing logcat process...', () async {
       final bool delivered = logcatProcess.kill(ProcessSignal.sigkill);
       assert(delivered);
+      await logcatProcessExitCode;
     });
 
     await step('Wait for Skia gold comparisons...', () async {

testing/scenario_app/bin/utils/process_manager_extension.dart

@@ -10,36 +10,25 @@ import 'package:process/process.dart';
 
 /// Pipes the [process] streams and writes them to [out] sink.
 /// If [out] is null, then the current [Process.stdout] is used as the sink.
-/// If [includePrefix] is true, then the prefix `[stdout]` or `[stderr]` is
-/// added before writting to the [out] sink.
 Future<int> pipeProcessStreams(
   Process process, {
   StringSink? out,
-  bool includePrefix = true,
 }) async {
   out ??= stdout;
   final Completer<void> stdoutCompleter = Completer<void>();
   final StreamSubscription<String> stdoutSub = process.stdout
     .transform(utf8.decoder)
     .transform<String>(const LineSplitter())
     .listen((String line) {
-      if (includePrefix) {
-        out!.writeln('[stdout] $line');
-      } else {
-        out!.writeln(line);
-      }
+      out!.writeln('[stdout] $line');
     }, onDone: stdoutCompleter.complete);
 
   final Completer<void> stderrCompleter = Completer<void>();
   final StreamSubscription<String> stderrSub = process.stderr
     .transform(utf8.decoder)
     .transform<String>(const LineSplitter())
     .listen((String line) {
-      if (includePrefix) {
-        out!.writeln('[stderr] $line');
-      } else {
-        out!.writeln(line);
-      }
+      out!.writeln('[stderr] $line');
     }, onDone: stderrCompleter.complete);
 
   final int exitCode = await process.exitCode;