Pass the flag in test_suite. I'm not super familiar with this code, but I'm

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
diff --git a/tools/testing/dart/test_suite.dart b/tools/testing/dart/test_suite.dart
index fd8270da32beb21a7ecd5a6b0d14beb5fee80779..183be62c326edfe62c34c80344dc51d57bc08f4b 100644
--- a/tools/testing/dart/test_suite.dart
+++ b/tools/testing/dart/test_suite.dart
@@ -2280,6 +2280,9 @@ class TestUtils {
     if (compiler == "dart2js" && configuration["cps_ir"]) {
       args.add("--use-cps-ir");
     }
+    if (compiler == "dart2js" && configuration["fast_startup"]) {
+      args.add("--fast-startup");
+    }
     if (compiler == "dart2analyzer") {
       args.add("--show-package-warnings");
       args.add("--enable-async");