fixes detection of SDK libraries

tool/input_sdk/private/js_helper.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.
 
-library _js_helper;
+library dart._js_helper;
 
 import 'dart:collection';
 
 import 'dart:_foreign_helper' show
     JS,
     JS_STRING_CONCAT;
 
 import 'dart:_interceptors';
 
 part 'annotations.dart';
@@ skipping 753 matching lines @@
 int random64() {
   // TODO(lrn): Use a secure random source.
   int int32a = JS("int", "(Math.random() * 0x100000000) >>> 0");
   int int32b = JS("int", "(Math.random() * 0x100000000) >>> 0");
   return int32a + int32b * 0x100000000;
 }
 
 String jsonEncodeNative(String string) {
   return JS("String", "JSON.stringify(#)", string);
 }