Avoid patching in dart2dart.

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 = "Computer says no!";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 2231 matching lines @@
       "No setter found for setter patch '#{setterName}'.");
 
   static const MessageKind PATCH_NON_CONSTRUCTOR = const MessageKind(
       "Cannot patch non-constructor with constructor patch "
       "'#{constructorName}'.");
 
   static const MessageKind PATCH_NON_FUNCTION = const MessageKind(
       "Cannot patch non-function with function patch "
       "'#{functionName}'.");
 
+  static const MessageKind EXTERNAL_WITH_BODY = const MessageKind(
+      "External function '#{functionName}' cannot have a function body.",
+      options: const ["--output-type=dart"],
+      howToFix: "Try removing the 'external' modifier or the function body.",
+      examples: const ["""
+external foo() => 0;
+main() => foo();
+""", """
+external foo() {}
+main() => foo();
+"""]);
+
   //////////////////////////////////////////////////////////////////////////////
   // Patch errors end.
   //////////////////////////////////////////////////////////////////////////////
 
   static const String IMPORT_EXPERIMENTAL_MIRRORS_PADDING = '\n*   ';
 
   static const MessageKind IMPORT_EXPERIMENTAL_MIRRORS =
       const MessageKind(r'''
 
 ****************************************************************
@@ skipping 75 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';
   }
 }