Fix concurrent modification error (#48916)

packages/flutter_tools/lib/src/ios/simulators.dart

@@ -809,7 +809,8 @@ class _IOSSimulatorDevicePortForwarder extends DevicePortForwarder {
 
   @override
   Future<void> dispose() async {
-    for (final ForwardedPort port in _ports) {
+    final List<ForwardedPort> portsCopy = List<ForwardedPort>.from(_ports);
+    for (final ForwardedPort port in portsCopy) {
       await unforward(port);
     }
   }

packages/flutter_tools/test/general.shard/ios/simulators_test.dart

@@ -44,6 +44,24 @@ void main() {
     osx.operatingSystem = 'macos';
   });
 
+  group('_IOSSimulatorDevicePortForwarder', () {
+    testUsingContext('dispose() does not throw an exception', () async {
+      final IOSSimulator simulator = IOSSimulator('123');
+      final DevicePortForwarder portForwarder = simulator.portForwarder;
+      await portForwarder.forward(123);
+      await portForwarder.forward(124);
+      expect(portForwarder.forwardedPorts.length, 2);
+      try {
+        await portForwarder.dispose();
+      } catch (e) {
+        fail('Encountered exception: $e');
+      }
+      expect(portForwarder.forwardedPorts.length, 0);
+    }, overrides: <Type, Generator>{
+      Platform: () => osx,
+    }, testOn: 'posix');
+  });
+
   group('logFilePath', () {
     testUsingContext('defaults to rooted from HOME', () {
       osx.environment['HOME'] = '/foo/bar';