Move js library back into compiler directory.

sdk/lib/_internal/compiler/js_lib/annotations.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.
 
 part of _js_helper;
 
 /// Tells the optimizing compiler that the annotated method has no
 /// side-effects.
 /// Requires @NoInline() to function correctly.
 class NoSideEffects {
@@ skipping 16 matching lines @@
 class IrRepresentation {
   final bool value;
   const IrRepresentation(this.value);
 }
 
 /// Marks a class as native and defines its JavaScript name(s).
 class Native {
   final String name;
   const Native(this.name);
 }