Move js library back into compiler directory.

sdk/lib/_internal/compiler/js_lib/js_mirrors.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_mirrors;
 
-import '../compiler/implementation/runtime_data.dart' as encoding;
+import '../implementation/runtime_data.dart' as encoding;
 
 import 'dart:collection' show
     UnmodifiableListView,
     UnmodifiableMapView;
 
 import 'dart:mirrors';
 
 import 'dart:_foreign_helper' show
     JS,
     JS_CURRENT_ISOLATE,
@@ skipping 2904 matching lines @@
   // have a part (following a '.') that starts with '_'.
   const int UNDERSCORE = 0x5f;
   if (name.isEmpty) return true;
   int index = -1;
   do {
     if (name.codeUnitAt(index + 1) == UNDERSCORE) return false;
     index = name.indexOf('.', index + 1);
   } while (index >= 0 && index + 1 < name.length);
   return true;
 }