[pkg/dartdev] opt a test out of nnbd

Bug: https://github.com/dart-lang/sdk/issues/44050
Change-Id: Id102c927ec4918940050783aa23aed0dad03b341
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170241
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

pkg/dartdev/test/commands/test_test.dart

@@ -85,6 +85,8 @@ void defineTest() {
   test('has package:test dependency', () {
     p = project(mainSrc: 'int get foo => 1;\n');
     p.file('test/foo_test.dart', '''
+$dartVersionFilePrefix2_9
+
 import 'package:test/test.dart';
 
 void main() {