dart2js: complain about missing part-of tags.

dart/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 275 matching lines @@
 
   static const VOID_NOT_ALLOWED = const MessageKind(
       'type void is only allowed in a return type.');
 
   static const BEFORE_TOP_LEVEL = const MessageKind(
       'Error: part header must come before top-level definitions.');
 
   static const LIBRARY_NAME_MISMATCH = const MessageKind(
       'Warning: expected part of library name "#{1}".');
 
+  static const MISSING_PART_OF_TAG = const MessageKind(
+      'Note: This file has no part-of tag, but it is being used as a part.');
+
   static const DUPLICATED_PART_OF = const MessageKind(
       'Error: duplicated part-of directive.');
 
   static const ILLEGAL_DIRECTIVE = const MessageKind(
       'Error: directive not allowed here.');
 
   static const DUPLICATED_LIBRARY_NAME = const MessageKind(
       'Warning: duplicated library name "#{1}".');
 
   static const INVALID_SOURCE_FILE_LOCATION = const MessageKind('''
@@ skipping 129 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);
 }