fixes detection of SDK libraries

tool/input_sdk/private/annotations.dart

 // Copyright (c) 2013, 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 _js_helper;
+part of dart._js_helper;
 
 /// Tells the optimizing compiler that the annotated method cannot throw.
 /// Requires @NoInline() to function correctly.
 class NoThrows {
   const NoThrows();
 }
 
 /// Tells the optimizing compiler to not inline the annotated method.
 class NoInline {
   const NoInline();
@@ skipping 22 matching lines @@
   /// matching the JavaScript calling conventions.
   final String name;
   const JsPeerInterface({this.name});
 }
 
 /// A Dart interface may only be implemented by a native JavaScript object
 /// if it is marked with this annotation.
 class SupportJsExtensionMethods {
   const SupportJsExtensionMethods();
 }