[dds][dart_runtime_service] Dynamically allocate ChromeDriver ports and fix SSE test flakiness

This change addresses flakiness in SSE tests caused by port conflicts
and race conditions during connection teardown.

Key changes:
- Dynamically allocates ChromeDriver ports in `sse_smoke_test.dart` and `sse_client_test.dart` instead of using a hardcoded port (4444). This avoids conflicts when tests run in parallel on CI bots.
- Adds support for locating ChromeDriver via the `CHROMEDRIVER_PATH` environment variable in both test suites.
- Updates `sse_smoke_driver.dart` to introduce a brief delay before closing connections. This ensures the client-side SseClient has sufficient time to flush its final POST request to the server, avoiding "Bad state: No element" errors.
- Modifies `test_helper.dart` to filter out `--mark_main_isolate_as_system_isolate` from arguments forwarded to spawned testee processes. This is necessary under modern `dart test` runners to ensure processes pause at start as expected.

Change-Id: Idf45cd69f366ec6b6f81cfed955aea337b277dcc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508343
Auto-Submit: Ben Konyi <bkonyi@google.com>
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Mark Zhou <markzipan@google.com>

pkg/dart_runtime_service/test/sse_client_test.dart

@@ -25,19 +25,27 @@ void main() {
   late HttpServer server;
   late WebDriver webdriver;
   late Process chromeDriver;
+  late int chromeDriverPort;
 
   setUpAll(() async {
+    final socket = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
+    chromeDriverPort = socket.port;
+    await socket.close();
+
     var chromedriverPath = Platform.environment['CHROMEDRIVER_PATH'];
+    final Uri chromedriverUri;
     if (chromedriverPath == null) {
       chromedriverPath = '../../../third_party/webdriver/chrome/chromedriver';
       if (Platform.isWindows) {
         chromedriverPath = '$chromedriverPath.exe';
       }
+      chromedriverUri = resolveTestRelativePath(chromedriverPath);
+    } else {
+      chromedriverUri = Uri.file(chromedriverPath);
     }
-    final chromedriverUri = resolveTestRelativePath(chromedriverPath);
     try {
       chromeDriver = await Process.start(chromedriverUri.toFilePath(), [
-        '--port=4444',
+        '--port=$chromeDriverPort',
         '--url-base=wd/hub',
       ]);
       final started = Completer<void>();
@@ -91,7 +99,10 @@ void main() {
             'binary': ?Platform.environment['CHROME_PATH'],
           },
         });
-      webdriver = await createDriver(desired: capabilities);
+      webdriver = await createDriver(
+        desired: capabilities,
+        uri: Uri.parse('http://localhost:$chromeDriverPort/wd/hub/'),
+      );
     });
 
     tearDown(() async {

pkg/dart_runtime_service/test/web/sse_smoke_driver.dart

@@ -23,9 +23,11 @@ Future<void> main() async {
     final client = SseClient(serviceUri);
     await client.onConnected;
     channel.sink.add('Success');
+    await Future<void>.delayed(const Duration(milliseconds: 500));
     client.close();
   } catch (e) {
     channel.sink.add('Error: $e');
+    await Future<void>.delayed(const Duration(milliseconds: 500));
   }
   channel.close();
 }

pkg/dds/test/common/test_helper.dart

@@ -56,7 +56,7 @@ Future<io.Process> spawnDartProcess(
     if (pauseOnStart) '--pause-isolates-on-start',
     if (disableServiceAuthCodes) '--disable-service-auth-codes',
     '--write-service-info=$serviceInfoUri',
-    ...io.Platform.executableArguments,
+    ...io.Platform.executableArguments.where((arg) => arg != '--mark_main_isolate_as_system_isolate'),
     resolveTestRelativePath(script).toFilePath(),
   ];
   final process = await io.Process.start(executable, arguments);

pkg/dds/test/sse_smoke_test.dart

@@ -22,6 +22,7 @@ import 'common/test_helper.dart';
 
 void main() {
   late Process chromeDriver;
+  late int chromeDriverPort;
   late DartDevelopmentService dds;
   late SseHandler handler;
   Process? process;
@@ -29,14 +30,24 @@ void main() {
   late WebDriver webdriver;
 
   setUpAll(() async {
-    var chromedriverPath = '../../../third_party/webdriver/chrome/chromedriver';
-    if (Platform.isWindows) {
-      chromedriverPath = '$chromedriverPath.exe';
+    final socket = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
+    chromeDriverPort = socket.port;
+    await socket.close();
+
+    var chromedriverPath = Platform.environment['CHROMEDRIVER_PATH'];
+    final Uri chromedriverUri;
+    if (chromedriverPath == null) {
+      chromedriverPath = '../../../third_party/webdriver/chrome/chromedriver';
+      if (Platform.isWindows) {
+        chromedriverPath = '$chromedriverPath.exe';
+      }
+      chromedriverUri = resolveTestRelativePath(chromedriverPath);
+    } else {
+      chromedriverUri = Uri.file(chromedriverPath);
     }
-    final chromedriverUri = resolveTestRelativePath(chromedriverPath);
     try {
       chromeDriver = await Process.start(chromedriverUri.toFilePath(), [
-        '--port=4444',
+        '--port=$chromeDriverPort',
         '--url-base=wd/hub',
       ]);
     } catch (e) {
@@ -71,6 +82,7 @@ void main() {
         });
       webdriver = await createDriver(
         desired: capabilities,
+        uri: Uri.parse('http://localhost:$chromeDriverPort/wd/hub/'),
       );
     });