Report external without patch as an error.

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;
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
@@ skipping 351 matching lines @@
       "count on origin method '#{1}' (#{2}).");
 
   static const PATCH_OPTIONAL_PARAMETER_NAMED_MISMATCH = const MessageKind(
       "Optional parameters of origin and patch method '#{1}' must "
       "both be either named or positional.");
 
   static const PATCH_PARAMETER_MISMATCH = const MessageKind(
       "Patch method parameter '#{3}' doesn't match '#{2}' on origin method "
       "#{1}.");
 
+  static const EXTERNAL_WITHOUT_IMPLEMENTATION = const MessageKind(
+      "External method without an implementation.");
+
   static const TOP_LEVEL_VARIABLE_DECLARED_STATIC = const MessageKind(
       "Top-level variable cannot be declared static.");
 
   static const WRONG_NUMBER_OF_ARGUMENTS_FOR_ASSERT = const MessageKind(
       "Wrong number of arguments to assert. Should be 1, but given #{1}.");
 
   static const ASSERT_IS_GIVEN_NAMED_ARGUMENTS = const MessageKind(
       "assert takes no named arguments, but given #{1}.");
 
   static const MALFORMED_TYPE_REFERENCE = const MessageKind(
@@ skipping 96 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }