sdk files reorganization to make dart2js a proper package

sdk/lib/_internal/js_runtime/lib/mirrors_patch.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.
 
 // Patch library for dart:mirrors.
 
 import 'dart:_js_helper' show patch;
 import 'dart:_js_mirrors' as js;
 
 @patch
@@ skipping 25 matching lines @@
   return (tm as ClassMirror).originalDeclaration;
 }
 
 @patch
 TypeMirror reflectType(Type key) {
   if (key == dynamic) {
     return currentMirrorSystem().dynamicType;
   }
   return js.reflectType(key);
 }