Enable conditional directives by default.

pkg/compiler/lib/src/commandline_options.dart

 // Copyright (c) 2015, 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 dart2js.cmdline.options;
 
 /// Commandline flags used in `dart2js.dart` and/or `apiimpl.dart`.
 class Flags {
   static const String allowMockCompilation = '--allow-mock-compilation';
   static const String allowNativeExtensions = '--allow-native-extensions';
@@ skipping 30 matching lines @@
   static const String testMode = '--test-mode';
   static const String trustPrimitives = '--trust-primitives';
   static const String trustTypeAnnotations = '--trust-type-annotations';
   static const String trustJSInteropTypeAnnotations =
       '--experimental-trust-js-interop-type-annotations';
   static const String useContentSecurityPolicy = '--csp';
   static const String useCpsIr = '--use-cps-ir';
   static const String useNewSourceInfo = '--use-new-source-info';
   static const String verbose = '--verbose';
   static const String version = '--version';
-
-  // Experimental flags.
-  static const String conditionalDirectives = '--conditional-directives';
 }
 
 class Option {
   static const String showPackageWarnings =
       '${Flags.showPackageWarnings}|${Flags.showPackageWarnings}=.*';
 }