Remove support for missing part-of tag.

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;
 
 /**
  * The messages in this file should meet the following guide lines:
  *
  * 1. The message should start with exactly one of "Error", "Internal error",
@@ skipping 516 matching lines @@
   static const MessageKind VOID_NOT_ALLOWED = const MessageKind(
       'Error: Type "void" is only allowed in a return type.');
 
   static const MessageKind BEFORE_TOP_LEVEL = const MessageKind(
       'Error: Part header must come before top-level definitions.');
 
   static const MessageKind LIBRARY_NAME_MISMATCH = const MessageKind(
       'Warning: Expected part of library name "#{libraryName}".');
 
   static const MessageKind MISSING_PART_OF_TAG = const MessageKind(
-      'Note: This file has no part-of tag, but it is being used as a part.');
+      'Error: This file has no part-of tag, but it is being used as a part.');
 
   static const MessageKind DUPLICATED_PART_OF = const MessageKind(
       'Error: Duplicated part-of directive.');
 
   static const MessageKind ILLEGAL_DIRECTIVE = const MessageKind(
       'Error: Directive not allowed here.');
 
   static const MessageKind DUPLICATED_LIBRARY_NAME = const MessageKind(
       'Warning: Duplicated library name "#{libraryName}".');
 
@@ skipping 264 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }