Using 'interface' and 'abstract' for methods should produce error

pkg/intl/lib/src/intl_helpers.dart

 // Copyright (c) 2012, 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.
 
 /**
  * A library for general helper code associated with the intl library
  * rather than confined to specific parts of it.
  */
 
 #library("intl_helpers");
@@ skipping 21 matching lines @@
 class LocaleDataException implements Exception {
   final String message;
   LocaleDataException(this.message);
   toString() => "LocaleDataException: $message";
 }
 
 /**
  *  An abstract superclass for data readers to keep the type system happy.
  */
 abstract class LocaleDataReader {
-  abstract Future read(String locale);
+  Future read(String locale);
 }
 
 /**
  * The internal mechanism for looking up messages. We expect this to be set
  * by the implementing package so that we're not dependent on its
  * implementation.
  */
 var messageLookup = const
     UninitializedLocaleData('initializeMessages(<locale>)');
 
 /**
  * Initialize the message lookup mechanism. This is for internal use only.
  * User applications should import `message_lookup_local.dart` and call
  * `initializeMessages`
  */
 void initializeInternalMessageLookup(Function lookupFunction) {
   if (messageLookup is UninitializedLocaleData) {
     messageLookup = lookupFunction();
   }
 }