Fix test.py and tests to use an explicit --packages flag.

pkg/analysis_server/test/integration/integration_tests.dart

Index: pkg/analysis_server/test/integration/integration_tests.dart
diff --git a/pkg/analysis_server/test/integration/integration_tests.dart b/pkg/analysis_server/test/integration/integration_tests.dart
index 1893c18fe23103f32522c669ec35cd0c9124a591..f6748eda6b39b3b9a2de7ec938781799f6043973 100644
--- a/pkg/analysis_server/test/integration/integration_tests.dart
+++ b/pkg/analysis_server/test/integration/integration_tests.dart
@@ -636,6 +636,9 @@ class Server {
     if (Platform.packageRoot != null) {
       arguments.add('--package-root=${Platform.packageRoot}');
     }
+    if (Platform.packageConfig != null) {
+      arguments.add('--packages=${Platform.packageConfig}');
+    }
     if (checked) {
       arguments.add('--checked');
     }