Do not encode the server port in the test file (#15535)

* Do not encode the server port in the test file

* revert formatting

* add back ignore

* add a default value to serverPort

* address comments

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -293,6 +293,7 @@ class _FlutterPlatform extends PlatformPlugin {
         startPaused: startPaused,
         bundlePath: _getBundlePath(finalizers, ourTestCount),
         observatoryPort: explicitObservatoryPort,
+        serverPort: server.port,
       );
       subprocessActive = true;
       finalizers.add(() async {
@@ -511,7 +512,7 @@ class _FlutterPlatform extends PlatformPlugin {
     listenerFile.createSync();
     listenerFile.writeAsStringSync(_generateTestMain(
       testUrl: fs.path.toUri(fs.path.absolute(testPath)).toString(),
-      encodedWebsocketUrl: Uri.encodeComponent(_getWebSocketUrl(server))
+      encodedWebsocketUrl: Uri.encodeComponent(_getWebSocketUrl()),
     ));
     return listenerFile.path;
   }
@@ -550,10 +551,10 @@ class _FlutterPlatform extends PlatformPlugin {
     return tempBundleDirectory.path;
   }
 
-  String _getWebSocketUrl(HttpServer server) {
+  String _getWebSocketUrl() {
     return host.type == InternetAddressType.IP_V4
-        ? 'ws://${host.address}:${server.port}'
-        : 'ws://[${host.address}]:${server.port}';
+        ? 'ws://${host.address}'
+        : 'ws://[${host.address}]';
   }
 
   String _generateTestMain({
@@ -576,7 +577,8 @@ import '$testUrl' as test;
 
 void main() {
   print('$_kStartTimeoutTimerMessage');
-  String server = Uri.decodeComponent('$encodedWebsocketUrl');
+  String serverPort = Platform.environment['SERVER_PORT'];
+  String server = Uri.decodeComponent('$encodedWebsocketUrl:\$serverPort');
   StreamChannel channel = serializeSuite(() {
     catchIsolateErrors();
     return test.main;
@@ -621,6 +623,7 @@ void main() {
     bool enableObservatory: false,
     bool startPaused: false,
     int observatoryPort,
+    int serverPort,
   }) {
     assert(executable != null); // Please provide the path to the shell in the SKY_SHELL environment variable.
     assert(!startPaused || enableObservatory);
@@ -660,6 +663,7 @@ void main() {
     final Map<String, String> environment = <String, String>{
       'FLUTTER_TEST': 'true',
       'FONTCONFIG_FILE': _fontConfigFile.path,
+      'SERVER_PORT': serverPort.toString(),
     };
     return processManager.start(command, environment: environment);
   }