sdk files reorganization to make dart2js a proper package

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;
-  }
-}