Close the exit port used by the compute isolate utility function (#86591)

packages/flutter/lib/src/foundation/_isolates_io.dart

@@ -59,6 +59,7 @@ Future<R> compute<Q, R>(isolates.ComputeCallback<Q, R> callback, Q message, { St
   await result.future;
   Timeline.startSync('$debugLabel: end', flow: Flow.end(flow.id));
   resultPort.close();
+  exitPort.close();
   errorPort.close();
   isolate.kill();
   Timeline.finishSync();

packages/flutter/test/foundation/_compute_caller.dart

+// 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.
+
+// A test script that invokes compute() to start an isolate.
+
+import 'package:flutter/src/foundation/_isolates_io.dart';
+
+int getLength(String s) {
+  return s.length;
+}
+
+Future<void> main() async {
+  const String s = 'hello world';
+  final int result = await compute(getLength, s);
+  if (result != s.length) {
+    throw Exception('compute returned bad result');
+  }
+}

packages/flutter/test/foundation/isolates_test.dart

@@ -2,8 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:io';
+import 'package:file/file.dart';
+import 'package:file/local.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:platform/platform.dart';
 
 int test1(int value) {
   return value + 1;
@@ -29,4 +33,18 @@ void main() {
     expect(await compute(test1Async, 0), 1);
     expect(compute(test2Async, 0), throwsException);
   }, skip: kIsWeb);
+
+  test('compute closes all ports', () async {
+    // Run a Dart script that calls compute().
+    // The Dart process will terminate only if the script exits cleanly with
+    // all isolate ports closed.
+    const FileSystem fs = LocalFileSystem();
+    const Platform platform = LocalPlatform();
+    final String flutterRoot = platform.environment['FLUTTER_ROOT']!;
+    final String dartPath = fs.path.join(flutterRoot, 'bin', 'cache', 'dart-sdk', 'bin', 'dart');
+    final String packageRoot = fs.path.dirname(fs.path.fromUri(platform.script));
+    final String scriptPath = fs.path.join(packageRoot, 'test', 'foundation', '_compute_caller.dart');
+    final ProcessResult result = await Process.run(dartPath, <String>[scriptPath]);
+    expect(result.exitCode, 0);
+  }, skip: kIsWeb);
 }