Remove backends.

compiler/java/com/google/dart/compiler/backend/js/ast/JsExpression.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;
-
-import com.google.dart.compiler.common.SourceInfo;
-
-/**
- * An abstract base class for all JavaScript expressions.
- */
-public abstract class JsExpression extends JsNode {
-
-  protected JsExpression() {
-  }
-
-  /**
-   * Determines whether the expression can cause side effects.
-   */
-  public abstract boolean hasSideEffects();
-
-  /**
-   * True if the target expression is definitely not null.
-   */
-  public abstract boolean isDefinitelyNotNull();
-
-  /**
-   * True if the target expression is definitely null.
-   */
-  public abstract boolean isDefinitelyNull();
-
-  /**
-   * Determines whether or not this expression is a leaf, such as a
-   * {@link JsNameRef}, {@link JsBooleanLiteral}, and so on. Leaf expressions
-   * never need to be parenthesized.
-   */
-  public boolean isLeaf() {
-    // Conservatively say that it isn't a leaf.
-    // Individual subclasses can speak for themselves if they are a leaf.
-    return false;
-  }
-
-  public JsExprStmt makeStmt() {
-    return new JsExprStmt(this);
-  }
-
-  @Override
-  public JsExpression setSourceRef(SourceInfo info) {
-    super.setSourceRef(info);
-    return this;
-  }
-}