dart2js: provide a better error-message, when a backend doesn't support mirrors.

pkg/compiler/lib/src/diagnostics/messages.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.
 
 
 /**
  * The messages in this file should meet the following guide lines:
  *
  * 1. The message should be a complete sentence starting with an uppercase
  * letter, and ending with a period.
@@ skipping 3334 matching lines @@
 Your app imports the unsupported library '#{uri}' via:
 ''''''
 $DISALLOWED_LIBRARY_IMPORT_PADDING#{importChain}
 
 Use the --categories option to support import of '#{uri}'.
 '''),
 
       MessageKind.MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND:
         const MessageTemplate(
           MessageKind.MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND,
-          "dart:mirrors library is not supported when using this backend."),
+          """
+dart:mirrors library is not supported when using this backend.
+
+Your app imports dart:mirrors via:""""""
+$MIRRORS_NOT_SUPPORTED_BY_BACKEND_PADDING#{importChain}"""),
 
       MessageKind.CALL_NOT_SUPPORTED_ON_NATIVE_CLASS:
         const MessageTemplate(MessageKind.CALL_NOT_SUPPORTED_ON_NATIVE_CLASS,
           "Non-supported 'call' member on a native class, or a "
           "subclass of a native class."),
 
       MessageKind.DIRECTLY_THROWING_NSM:
         const MessageTemplate(MessageKind.DIRECTLY_THROWING_NSM,
           "This 'noSuchMethod' implementation is guaranteed to throw an "
           "exception. The generated code will be smaller if it is "
@@ skipping 20 matching lines @@
           "Unsupported version of package:lookup_map.",
           howToFix: DONT_KNOW_HOW_TO_FIX),
 
   }; // End of TEMPLATES.
 
   /// Padding used before and between import chains in the message for
   /// [MessageKind.IMPORT_EXPERIMENTAL_MIRRORS].
   static const String IMPORT_EXPERIMENTAL_MIRRORS_PADDING = '\n*   ';
 
   /// Padding used before and between import chains in the message for
+  /// [MessageKind.MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND].
+  static const String MIRRORS_NOT_SUPPORTED_BY_BACKEND_PADDING = '\n   ';
+
+  /// Padding used before and between import chains in the message for
   /// [MessageKind.DISALLOWED_LIBRARY_IMPORT].
   static const String DISALLOWED_LIBRARY_IMPORT_PADDING = '\n  ';
 
   toString() => template;
 
   Message message([Map arguments = const {}, bool terse = false]) {
     return new Message(this, arguments, terse);
   }
 
   bool get hasHowToFix => howToFix != null && howToFix != DONT_KNOW_HOW_TO_FIX;
@@ 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';
   }
 }