Remove backends.

compiler/java/com/google/dart/compiler/backend/js/ast/JsContext.java

-// Copyright (c) 2011, 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.
-
-package com.google.dart.compiler.backend.js.ast;
-
-/**
- * The context in which a JsNode visitation occurs. This represents the set of
- * possible operations a JsVisitor subclass can perform on the currently visited
- * node.
- */
-public interface JsContext {
-
-  boolean canInsert();
-
-  boolean canRemove();
-
-  void insertAfter(JsVisitable node);
-
-  void insertBefore(JsVisitable node);
-
-  boolean isLvalue();
-
-  void removeMe();
-
-  void replaceMe(JsVisitable node);
-}