Emit warning on import of dart:mirrors.

tests/compiler/dart2js/mirrors_used_test.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.
 
 /// Test that the @MirrorsUsed annotation suppress hints and that only
 /// requested elements are retained for reflection.
 library dart2js.test.mirrors_used_test;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 27 matching lines @@
   if (kind.name == 'info') return;
 
   // TODO(aprelev@gmail.com): Remove once dartbug.com/13907 is fixed.
   if (message.contains("Warning: 'typedef' not allowed here")) return;
 
   throw '$uri:$begin:$end: $kind: $message';
 }
 
 void main() {
   Compiler compiler = compilerFor(
-      MEMORY_SOURCE_FILES, diagnosticHandler: expectOnlyVerboseInfo);
+      MEMORY_SOURCE_FILES, diagnosticHandler: expectOnlyVerboseInfo,
+      options: ['--enable-experimental-mirrors']);
   asyncTest(() => compiler.runCompiler(Uri.parse('memory:main.dart')).then((_) {
     print('');
     List generatedCode =
         Elements.sortedByPosition(compiler.enqueuer.codegen.generatedCode.keys);
     for (var element in generatedCode) {
       print(element);
     }
     print('');
 
     // This assertion can fail for two reasons:
@@ skipping 106 matching lines @@
 library lib;
 
 import 'dart:mirrors';
 
 useReflect(type) {
   print(new Symbol('Foo'));
   print(MirrorSystem.getName(reflectClass(type).owner.qualifiedName));
 }
 """,
 };