Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/apiimpl.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.
 
 library leg_apiimpl;
 
 import 'dart:async';
 
 import '../compiler.dart' as api;
 import 'dart2jslib.dart' as leg;
@@ skipping 62 matching lines @@
             buildId: extractStringOption(
                 options, '--build-id=',
                 "build number could not be determined"),
             showPackageWarnings:
                 hasOption(options, '--show-package-warnings'),
             useContentSecurityPolicy: hasOption(options, '--csp'),
             hasIncrementalSupport:
                 forceIncrementalSupport ||
                 hasOption(options, '--incremental-support'),
             suppressWarnings: hasOption(options, '--suppress-warnings'),
+            enableExperimentalMirrors:

[rmacnak, 2015/01/14 18:30:29]

Uhh, what do you intend to put behind this flag?

[floitsch, 2015/01/15 10:43:52]

The implementation of dart:mirrors in dart2js is gated by this flag.

[rmacnak, 2015/01/15 21:48:52]

The existing implementation? That's quite a significant breaking change.

[floitsch, 2015/01/15 22:35:15]

If adding a flag to a compilation process is a significant breaking change, then
yes.
Fwiw it's not going to happen in 1.9.

+                hasOption(options, '--enable-experimental-mirrors'),
             enableAsyncAwait: hasOption(options, '--enable-async'),
             enableEnums: hasOption(options, '--enable-enum')) {
     tasks.addAll([
         userHandlerTask = new leg.GenericTask('Diagnostic handler', this),
         userProviderTask = new leg.GenericTask('Input provider', this),
     ]);
     if (!libraryRoot.path.endsWith("/")) {
       throw new ArgumentError("libraryRoot must end with a /");
     }
     if (packageRoot != null && !packageRoot.path.endsWith("/")) {
@@ skipping 264 matching lines @@
   }
 
   void diagnoseCrashInUserCode(String message, exception, stackTrace) {
     hasCrashed = true;
     print('$message: ${tryToString(exception)}');
     print(tryToString(stackTrace));
   }
 
   fromEnvironment(String name) => environment[name];
 }