Report an error from dart2dart when deferred loading is used.

sdk/lib/_internal/compiler/implementation/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 1157 matching lines @@
 
   static const MessageKind FACTORY_REDIRECTION_IN_NON_FACTORY =
       const MessageKind(
           "Factory redirection only allowed in factories.");
 
   static const MessageKind MISSING_FACTORY_KEYWORD = const MessageKind(
       "Did you forget a factory keyword here?");
 
   static const MessageKind DEFERRED_LIBRARY_DART_2_DART =
       const MessageKind(
-          "Deferred loading is not supported by the dart backend yet. "
-          "Will not split the output.");
+          "Deferred loading is not supported by the dart backend yet.");
 
   static const MessageKind DEFERRED_LIBRARY_WITHOUT_PREFIX =
       const MessageKind(
           "This import is deferred but there is no prefix keyword.",
           howToFix:
             "Try adding a prefix to the import.");
 
   static const MessageKind DEFERRED_LIBRARY_DUPLICATE_PREFIX =
       const MessageKind(
           "The prefix of this deferred import is not unique.",
@@ skipping 891 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';
   }
 }