Fix a test which was broken by the previous revision.

test/runner/runner_test.dart

Index: test/runner/runner_test.dart
diff --git a/test/runner/runner_test.dart b/test/runner/runner_test.dart
index d717b671d62247cc2ec7add3102b85ca989c50f9..2aed93d54bc8823853bfccbae5075eefd78b0ae3 100644
--- a/test/runner/runner_test.dart
+++ b/test/runner/runner_test.dart
@@ -151,7 +151,7 @@ $_usage"""));
         contains('-1: load error'),
         contains(
           '  Failed to load "${p.relative(testPath, from: _sandbox)}":\n'
-          "  Error on line 1, column 8: TestOn takes one argument.\n"
+          "  Error on line 1, column 8: TestOn takes 1 argument.\n"
           "  @TestOn()\n"
           "         ^^\n")
       ]));