Fix old bug in the test runner's test server.

I apparently broke this four years ago (in
https://codereview.chromium.org/2919573003) and never realized. Caught
it today because of a lint warning.

Change-Id: I00d71ccde581e685620800b603b0ffb9e8e3a1b7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/192951
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Devon Carew <devoncarew@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>
diff --git a/pkg/test_runner/lib/src/testing_servers.dart b/pkg/test_runner/lib/src/testing_servers.dart
index 5e9df68..bbd5cfd 100644
--- a/pkg/test_runner/lib/src/testing_servers.dart
+++ b/pkg/test_runner/lib/src/testing_servers.dart
@@ -343,7 +343,7 @@
       ]) {
         response.headers.set(header, content_header_value);
       }
-      if (const ["safari"].contains(runtime)) {
+      if (runtime == Runtime.safari) {
         response.headers.set("X-WebKit-CSP", content_header_value);
       }
     }