[ package:vm_service ] Add missing code to code generator

Was added originally in https://dart-review.googlesource.com/c/sdk/+/367821

Change-Id: I7873e960319c10e995e24b1bb2514ede0c3299d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/371360
Commit-Queue: Ben Konyi <bkonyi@google.com>
Reviewed-by: Derek Xu <derekx@google.com>
Commit-Queue: Derek Xu <derekx@google.com>
Auto-Submit: Ben Konyi <bkonyi@google.com>
diff --git a/pkg/vm_service/tool/dart/generate_dart_client.dart b/pkg/vm_service/tool/dart/generate_dart_client.dart
index 986630d..51d1f3a 100644
--- a/pkg/vm_service/tool/dart/generate_dart_client.dart
+++ b/pkg/vm_service/tool/dart/generate_dart_client.dart
@@ -100,6 +100,13 @@
   }
 
   Future<T> _call<T>(String method, [Map args = const {}]) {
+    if (_disposed) {
+      throw RPCError(
+        method,
+        RPCErrorKind.kServerError.code,
+        'Service connection disposed',
+      );
+    }
     return wrapFuture<T>(
       method,
       () {