[flutter_tools] Fix hang in DesktopLogReader (#88178)

packages/flutter_tools/lib/src/desktop_device.dart

@@ -314,15 +314,15 @@ class DesktopLogReader extends DeviceLogReader {
     final StreamSubscription<List<int>> stderrSub = process.stderr.listen(
       _inputController.add,
     );
+    final Future<void> stdioFuture = Future.wait<void>(<Future<void>>[
+      stdoutSub.asFuture<void>(),
+      stderrSub.asFuture<void>(),
+    ]);
     process.exitCode.whenComplete(() async {
       // Wait for output to be fully processed.
-      await Future.wait<void>(<Future<void>>[
-        stdoutSub.asFuture<void>(),
-        stderrSub.asFuture<void>(),
-      ]);
-      // The streams as futures have already completed, so waiting for the
-      // potentially async stream cancellation to complete likely has no
-      // benefit.
+      await stdioFuture;
+      // The streams have already completed, so waiting for the stream
+      // cancellation to complete is not needed.
       unawaited(stdoutSub.cancel());
       unawaited(stderrSub.cancel());
       await _inputController.close();

packages/flutter_tools/test/general.shard/desktop_device_test.dart

@@ -289,17 +289,13 @@ void main() {
     final FakeDesktopDevice device = setUpDesktopDevice(
       processManager: processManager,
     );
-    final Completer<void> testCompleter = Completer<void>();
-    final List<String> logOutput = <String>[];
-    device.getLogReader().logLines.listen((String line) {
-      logOutput.add(line);
-    }, onDone: () {
-      expect(logOutput, contains('Oops'));
-      testCompleter.complete();
-    });
     unawaited(Future<void>(() {
       exitCompleter.complete();
     }));
+
+    // Start looking for 'Oops' in the stream before starting the app.
+    expect(device.getLogReader().logLines, emits('Oops'));
+
     final FakeApplicationPackage package = FakeApplicationPackage();
     await device.startApp(
       package,
@@ -309,7 +305,6 @@ void main() {
         dartEntrypointArgs: <String>['arg1', 'arg2'],
       ),
     );
-    await testCompleter.future;
   });
 }