Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 314 matching lines @@
     new OptionHandler('--analyze-signatures-only', setAnalyzeOnly),
     new OptionHandler('--disable-native-live-type-analysis', passThrough),
     new OptionHandler('--categories=.*', setCategories),
     new OptionHandler('--disable-type-inference', implyCompilation),
     new OptionHandler('--terse', passThrough),
     new OptionHandler('--dump-info', implyCompilation),
     new OptionHandler('--disallow-unsafe-eval',
                       (_) => hasDisallowUnsafeEval = true),
     new OptionHandler('--show-package-warnings', passThrough),
     new OptionHandler('--csp', passThrough),
+    new OptionHandler('--enable-experimental-mirrors', passThrough),
     new OptionHandler('--enable-async', setEnableAsync),
     new OptionHandler('--enable-enum', passThrough),
     new OptionHandler('-D.+=.*', addInEnvironment),
 
     // The following two options must come last.
     new OptionHandler('-.*', (String argument) {
       helpAndFail("Unknown option '$argument'.");
     }),
     new OptionHandler('.*', (String argument) {
       arguments.add(nativeToUriPath(argument));
@@ skipping 344 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }