Move js library back into compiler directory.

sdk/lib/_internal/compiler/js_lib/mirror_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.
 /**
  * Helps dealing with reflection in the case that the source code has been
  * changed as a result of compiling with dart2dart.
  */
 library _mirror_helper;
 
 import 'dart:mirrors';
 
 /// The compiler will replace this variable with a map containing all the
 /// renames made in dart2dart.
 const Map<String, String> _SYMBOLS = null;
 
 /// This method is a wrapper for MirrorSystem.getName() and will be inlined and
 /// called in the generated output Dart code.
 String helperGetName(Symbol sym) {
   var name = MirrorSystem.getName(sym);
   if (_SYMBOLS.containsKey(name)) {
     return _SYMBOLS[name];
   } else {
     return name;
   }
 }