Move js library back into compiler directory.

sdk/lib/_internal/compiler/js_lib/js_names.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 dart._js_names;
 
 import 'dart:_foreign_helper' show
     JS,
     JS_GET_NAME;
 
@@ skipping 86 matching lines @@
   var names = JS('=Object', 'init.mangledGlobalNames');
   return JsCache.fetch(names, name);
 }
 
 String unmangleAllIdentifiersIfPreservedAnyways(String str) {
   return JS("String",
             r"(#).replace(/[^<,> ]+/g,"
             r"function(m) { return init.mangledGlobalNames[m] || m; })",
             str);
 }