Add web socket command to stop serving a directory.

sdk/lib/_internal/pub/test/serve/web_socket/unserve_directory_not_served_test.dart

Index: sdk/lib/_internal/pub/test/serve/web_socket/unserve_directory_not_served_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/web_socket/serve_directory_already_served_test.dart b/sdk/lib/_internal/pub/test/serve/web_socket/unserve_directory_not_served_test.dart
similarity index 68%
copy from sdk/lib/_internal/pub/test/serve/web_socket/serve_directory_already_served_test.dart
copy to sdk/lib/_internal/pub/test/serve/web_socket/unserve_directory_not_served_test.dart
index a3c8f0ce84b36f9f8089ef122b50b58c0a8a9734..7d80601aecfd6f9f81b61566a502175b206e953a 100644
--- a/sdk/lib/_internal/pub/test/serve/web_socket/serve_directory_already_served_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/web_socket/unserve_directory_not_served_test.dart
@@ -10,7 +10,7 @@ import '../utils.dart';
 
 main() {
   initConfig();
-  integration("returns the old URL if the directory is already served", () {
+  integration("errors if the directory is not served", () {
     d.dir(appPath, [
       d.appPubspec(),
       d.dir("web", [
@@ -20,11 +20,13 @@ main() {
 
     pubServe();
 
+    // Unbind the directory.
     expectWebSocketCall({
-      "command": "serveDirectory",
-      "path": "web"
-    }, replyMatches: {
-      "url": getServerUrl("web")
+      "command": "unserveDirectory",
+      "path": "test"
+    }, replyEquals: {
+      "code": "NOT_SERVED",
+      "error": 'Directory "test" is not bound to a server.'
     });
 
     endPubServe();