Remove backends.

compiler/java/com/google/dart/compiler/backend/js/ast/JsStatement.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;
-
-/**
- * Abstract base class for JavaScript statement objects.
- */
-public abstract class JsStatement extends JsNode {
-
-  protected JsStatement() {
-  }
-
-  /**
-   * Returns true if this statement definitely causes an abrupt change in flow
-   * control.
-   */
-  public boolean unconditionalControlBreak() {
-    return false;
-  }
-}