fixes detection of SDK libraries

tool/input_sdk/private/interceptors.dart

 // Copyright (c) 2012, 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.
 
-library _interceptors;
+library dart._interceptors;
 
 import 'dart:collection';
 import 'dart:_internal' hide Symbol;
 import 'dart:_js_helper';
 import 'dart:_foreign_helper' show JS;
 import 'dart:math' show Random;
 
 part 'js_array.dart';
 part 'js_number.dart';
 part 'js_string.dart';
@@ skipping 79 matching lines @@
  * Interceptor for unclassified JavaScript objects, typically objects with a
  * non-trivial prototype chain.
  *
  * This class also serves as a fallback for unknown JavaScript exceptions.
  */
 class UnknownJavaScriptObject extends JavaScriptObject {
   const UnknownJavaScriptObject();
 
   String toString() => JS('String', 'String(#)', this);
 }