Close receive port in VM platform on null isolate (#2665)

This resolves flakiness in top_level_configuration_test.dart

pkgs/test_core/lib/src/runner/vm/platform.dart

diff --git a/pkgs/test_core/lib/src/runner/vm/platform.dart b/pkgs/test_core/lib/src/runner/vm/platform.dart
index 0f7620e..8916e77 100644
--- a/pkgs/test_core/lib/src/runner/vm/platform.dart
+++ b/pkgs/test_core/lib/src/runner/vm/platform.dart
@@ -87,7 +87,10 @@
           suiteConfig.metadata,
           platform.compiler,
         );
-        if (isolate == null) return null;
+        if (isolate == null) {
+          receivePort.close();
+          return null;
+        }
       } catch (error) {
         receivePort.close();
         rethrow;