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

tools/testing/dart/compiler_configuration.dart

Index: tools/testing/dart/compiler_configuration.dart
diff --git a/tools/testing/dart/compiler_configuration.dart b/tools/testing/dart/compiler_configuration.dart
index e008e71adb24544e72abb50f7ceee5b2cc406569..d0e6f5f444ad02c68cd67c3c56f7e8c084c9c4ad 100644
--- a/tools/testing/dart/compiler_configuration.dart
+++ b/tools/testing/dart/compiler_configuration.dart
@@ -303,9 +303,6 @@ class Dart2jsCompilerConfiguration extends Dart2xCompilerConfiguration {
       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)