Remove backends.

compiler/java/com/google/dart/compiler/Backend.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;
-
-import com.google.dart.compiler.ast.DartUnit;
-import com.google.dart.compiler.ast.LibraryUnit;
-import com.google.dart.compiler.resolver.CoreTypeProvider;
-
-import java.io.IOException;
-import java.util.Collection;
-
-/**
- * Interface for compiler backends.
- */
-public interface Backend {
-
-  /**
-   * Determines whether compilation artifacts are out of date with respect to
-   * this source.
-   */
-  boolean isOutOfDate(DartSource src, DartCompilerContext context);
-
-  /**
-   * Compile the given compilation unit.
-   * @param context The listener through which compilation errors are reported
-   *          (not <code>null</code>)
-   */
-  void compileUnit(DartUnit unit, DartSource src,
-                   DartCompilerContext context,
-                   CoreTypeProvider typeProvider)
-      throws IOException;
-
-  /**
-   * Package the given application.
-   *
-   * @param app The application library whose entry-point should be called
-   * @param libraries The transitive set of libraries contained in this
-   *          application
-   * @param context The listener through which compilation errors are reported
-   *          (not <code>null</code>)
-   */
-  void packageApp(LibrarySource app,
-                  Collection<LibraryUnit> libraries,
-                  DartCompilerContext context,
-                  CoreTypeProvider typeProvider)
-      throws IOException;
-
-  /**
-   * The application extension for the backend.
-   */
-  String getAppExtension();
-}