[dart] Make dart_test testonly.

Change-Id: I52a8599f63999980000d8f815d55746091690fb7
diff --git a/dart/dart_test.gni b/dart/dart_test.gni
index 7b7d76c..8a71fad 100644
--- a/dart/dart_test.gni
+++ b/dart/dart_test.gni
@@ -65,6 +65,8 @@
   action(main_target_name) {
     script = "//build/dart/gen_test_invocation.py"
 
+    testonly = true
+
     outputs = [
       invocation_file,
     ]