fixes detection of SDK libraries

tool/input_sdk/private/string_helper.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.
 
-part of _js_helper;
+part of dart._js_helper;
 
 class StringMatch implements Match {
   const StringMatch(int this.start,
                     String this.input,
                     String this.pattern);
 
   int get end => start + pattern.length;
   String operator[](int g) => group(g);
   int get groupCount => 0;
 
@@ skipping 190 matching lines @@
   } else {
     checkNull(from);
     // TODO(floitsch): implement generic String.replace (with patterns).
     throw "String.replace(Pattern) UNIMPLEMENTED";
   }
 }
 
 stringJoinUnchecked(array, separator) {
   return JS('String', r'#.join(#)', array, separator);
 }