[flutter_tools] disable fast-start on routes test (#49473)

dev/devicelab/bin/tasks/routing_test.dart

@@ -34,7 +34,8 @@ void main() {
       print('run: starting...');
       final Process run = await startProcess(
         path.join(flutterDirectory.path, 'bin', 'flutter'),
-        <String>['run', '--verbose', '--disable-service-auth-codes', '-d', device.deviceId, '--route', '/smuggle-it', 'lib/route.dart'],
+        // --fast-start does not support routes.
+        <String>['run', '--verbose', '--disable-service-auth-codes', '--no-fast-start', '-d', device.deviceId, '--route', '/smuggle-it', 'lib/route.dart'],
       );
       run.stdout
         .transform<String>(utf8.decoder)