Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 2208 matching lines @@
       "'#{constructorName}'.");
 
   static const MessageKind PATCH_NON_FUNCTION = const MessageKind(
       "Cannot patch non-function with function patch "
       "'#{functionName}'.");
 
   //////////////////////////////////////////////////////////////////////////////
   // Patch errors end.
   //////////////////////////////////////////////////////////////////////////////
 
+  static const String IMPORT_EXPERIMENTAL_MIRRORS_PADDING = '\n*   ';
+
+  static const MessageKind IMPORT_EXPERIMENTAL_MIRRORS =
+      const MessageKind(r'''
+
+****************************************************************
+* WARNING: dart:mirrors support in dart2js is experimental,
+*          and not recommended.
+*          This implementation of mirrors is incomplete,
+*          and often greatly increases the size of the generated
+*          JavaScript code.
+*
+* Your app imports dart:mirrors via:''''''
+$IMPORT_EXPERIMENTAL_MIRRORS_PADDING#{importChain}
+*
+* Starting with Dart 1.9, you must use the
+* --enable-experimental-mirrors command-line flag to opt-in.
+* You can begin using this flag now if mirrors support is critical.
+*
+* To learn what to do next, please visit:
+*    http://dartlang.org/dart2js-reflection
+****************************************************************
+''');
+
   static const MessageKind CALL_NOT_SUPPORTED_ON_NATIVE_CLASS =
       const MessageKind(
           "Non-supported 'call' member on a native class, or a "
           "subclass of a native class.");
 
   toString() => template;
 
   Message message([Map arguments = const {}, bool terse = false]) {
     return new Message(this, arguments, terse);
   }
@@ skipping 47 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }