Use finally block

If there is no exception and we never close the servers they can hold
the process open.

I'm not sure if this fully explains the flakiness on travis...

Author: natebosch

pkgs/test/lib/src/runner/node/platform.dart

@@ -124,9 +124,8 @@ class NodePlatform extends PlatformPlugin
       }));
 
       return Pair(channel, pair.last);
-    } catch (_) {
+    } finally {
       unawaited(Future.wait(servers.map((s) => s.close().catchError((_) {}))));
-      rethrow;
     }
   }