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

tools/testing/dart/test_suite.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 2262 matching lines @@
     }
     if ((compiler == "dart2js") && configuration["minified"]) {
       args.add("--minify");
     }
     if (compiler == "dart2js" && configuration["csp"]) {
       args.add("--csp");
     }
     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");
     }
     return args;
   }
 
   static bool isBrowserRuntime(String runtime) {
     const BROWSERS = const [
       'drt',
@@ skipping 179 matching lines @@
       }
       if (path.length > WINDOWS_SHORTEN_PATH_LIMIT) {
         ++shortNameCounter;
         var pathEnd = path.substring(path.length - WINDOWS_PATH_END_LENGTH);
         path = "short${shortNameCounter}_$pathEnd";
       }
     }
     return path;
   }
 }