Fix manual starting of test runner HTTP servers

tools/testing/dart/http_server.dart

Index: tools/testing/dart/http_server.dart
diff --git a/tools/testing/dart/http_server.dart b/tools/testing/dart/http_server.dart
index c1ef6cb8104cc35cf4414ae8c0e6523955304b36..4e6889403bae70ec851431c41e3e3d7b296c520b 100644
--- a/tools/testing/dart/http_server.dart
+++ b/tools/testing/dart/http_server.dart
@@ -71,9 +71,10 @@ main() {
     var crossOriginPort = int.parse(args['crossOriginPort']);
     servers.startServers(args['network'],
                          port: port,
-                         crossOriginPort: crossOriginPort);
-    DebugLogger.info('Server listening on port ${servers.port}');
-    DebugLogger.info('Server listening on port ${servers.crossOriginPort}');
+                         crossOriginPort: crossOriginPort).then((_) {
+      DebugLogger.info('Server listening on port ${servers.port}');
+      DebugLogger.info('Server listening on port ${servers.crossOriginPort}');
+    });
   }
 }
 
@@ -102,10 +103,10 @@ class TestingServers {
    *   "Access-Control-Allow-Credentials: true"
    */
   Future startServers(String host, {int port: 0, int crossOriginPort: 0}) {
-    return _startHttpServer(host, port: port).then((_) {
-      _startHttpServer(host,
-                       port: crossOriginPort,
-                       allowedPort:_serverList[0].port);
+    return _startHttpServer(host, port: port).then((server) {
+      return _startHttpServer(host,
+                              port: crossOriginPort,
+                              allowedPort:_serverList[0].port);
     });
   }