Emit warning on import of dart:mirrors.

tests/compiler/dart2js/message_kind_helper.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.test.message_kind_helper;
 
 import 'package:expect/expect.dart';
 import 'dart:async';
 
 import 'package:compiler/src/dart2jslib.dart' show
@@ skipping 65 matching lines @@
     void collect(Uri uri, int begin, int end, String message, kind) {
       if (kind.name == 'verbose info' || kind.name == 'info') {
         return;
       }
       messages.add(message);
     }
 
     Compiler compiler = compilerFor(
         example,
         diagnosticHandler: collect,
-        options: ['--analyze-only']..addAll(kind.options),
+        options: ['--analyze-only',
+                  '--enable-experimental-mirrors']..addAll(kind.options),
         cachedCompiler: cachedCompiler);
 
     return compiler.run(Uri.parse('memory:main.dart')).then((_) {
 
       Expect.isFalse(messages.isEmpty, 'No messages in """$example"""');
 
       String expectedText = !kind.hasHowToFix
           ? kind.template : '${kind.template}\n${kind.howToFix}';
       String pattern = expectedText.replaceAllMapped(
           new RegExp(ESCAPE_REGEXP), (m) => '\\${m[0]}');
@@ skipping 41 matching lines @@
       Expect.isTrue(!pendingStuff || kindsWithPendingClasses.contains(kind));
 
       if (!pendingStuff && !compiler.compilerWasCancelled) {
         // If there is pending stuff, or the compiler was cancelled, we
         // shouldn't reuse the compiler.
         cachedCompiler = compiler;
       }
     });
   }).then((_) => cachedCompiler);
 }