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

tools/testing/dart/compiler_configuration.dart

 // Copyright (c) 2014, 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.
 
 library compiler_configuration;
 
 import 'dart:io' show Platform;
 
 import 'runtime_configuration.dart' show RuntimeConfiguration;
 
@@ skipping 285 matching lines @@
   }
 
   CommandArtifact computeCompilationArtifact(
       String buildDir,
       String tempDir,
       CommandBuilder commandBuilder,
       List arguments,
       Map<String, String> environmentOverrides) {
     List compilerArguments = new List.from(arguments)
       ..addAll(extraDart2jsOptions);
-    if (useFastStartup) {
-      compilerArguments.add('--fast-startup');
-    }
     return new CommandArtifact(<Command>[
       this.computeCompilationCommand('$tempDir/out.js', buildDir,
           CommandBuilder.instance, compilerArguments, environmentOverrides)
     ], '$tempDir/out.js', 'application/javascript');
   }
 
   List<String> computeRuntimeArguments(
       RuntimeConfiguration runtimeConfiguration,
       String buildDir,
       TestInformation info,
@@ skipping 352 matching lines @@
       RuntimeConfiguration runtimeConfiguration,
       String buildDir,
       TestInformation info,
       List<String> vmOptions,
       List<String> sharedOptions,
       List<String> originalArguments,
       CommandArtifact artifact) {
     return <String>[];
   }
 }