Expose Forwarding* as part of the main interface. (flutter#12)

As part of this, add ForwardingFileSystem.

Fixes flutter#2

lib/file.dart

@@ -4,4 +4,5 @@
 
 /// Core interfaces containing the abstract `FileSystem` interface definition
 /// and all associated types used by `FileSystem`.
+export 'src/forwarding.dart';
 export 'src/interface.dart';

lib/src/forwarding.dart

@@ -13,5 +13,6 @@ import 'package:meta/meta.dart';
 
 part 'forwarding/forwarding_directory.dart';
 part 'forwarding/forwarding_file.dart';
+part 'forwarding/forwarding_file_system.dart';
 part 'forwarding/forwarding_file_system_entity.dart';
 part 'forwarding/forwarding_link.dart';

lib/src/forwarding/forwarding_directory.dart

@@ -4,6 +4,7 @@
 
 part of file.src.forwarding;
 
+/// A directory that forwards all methods and properties to a delegate.
 abstract class ForwardingDirectory
     extends ForwardingFileSystemEntity<Directory, io.Directory>
     implements Directory {

lib/src/forwarding/forwarding_file.dart

@@ -4,6 +4,7 @@
 
 part of file.src.forwarding;
 
+/// A file that forwards all methods and properties to a delegate.
 abstract class ForwardingFile extends ForwardingFileSystemEntity<File, io.File>
     implements File {
   @override

lib/src/forwarding/forwarding_file_system.dart

@@ -0,0 +1,62 @@
+// Copyright (c) 2017, 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 file.src.forwarding;
+
+/// A file system that forwards all methods and properties to a delegate.
+abstract class ForwardingFileSystem extends FileSystem {
+  /// The file system to which this file system will forward all activity.
+  @protected
+  final FileSystem delegate;
+
+  /// Creates a new [ForwardingFileSystem] that forwards all methods and
+  /// properties to the specified [delegate].
+  ForwardingFileSystem(this.delegate);
+
+  @override
+  Directory directory(path) => delegate.directory(path);
+
+  @override
+  File file(path) => delegate.file(path);
+
+  @override
+  Link link(path) => delegate.link(path);
+
+  @override
+  String get pathSeparator => delegate.pathSeparator;
+
+  @override
+  Directory get systemTempDirectory => delegate.systemTempDirectory;
+
+  @override
+  Directory get currentDirectory => delegate.currentDirectory;
+
+  @override
+  set currentDirectory(dynamic path) => delegate.currentDirectory = path;
+
+  @override
+  Future<io.FileStat> stat(String path) => delegate.stat(path);
+
+  @override
+  io.FileStat statSync(String path) => delegate.statSync(path);
+
+  @override
+  Future<bool> identical(String path1, String path2) =>
+      delegate.identical(path1, path2);
+
+  @override
+  bool identicalSync(String path1, String path2) =>
+      delegate.identicalSync(path1, path2);
+
+  @override
+  bool get isWatchSupported => delegate.isWatchSupported;
+
+  @override
+  Future<io.FileSystemEntityType> type(String path, {bool followLinks: true}) =>
+      delegate.type(path, followLinks: followLinks);
+
+  @override
+  io.FileSystemEntityType typeSync(String path, {bool followLinks: true}) =>
+      delegate.typeSync(path, followLinks: followLinks);
+}

lib/src/forwarding/forwarding_file_system_entity.dart

@@ -4,8 +4,10 @@
 
 part of file.src.forwarding;
 
+/// A file system entity that forwards all methods and properties to a delegate.
 abstract class ForwardingFileSystemEntity<T extends FileSystemEntity,
     D extends io.FileSystemEntity> implements FileSystemEntity {
+  /// The entity to which this entity will forward all methods and properties.
   @protected
   D get delegate;
 

lib/src/forwarding/forwarding_link.dart

@@ -4,6 +4,7 @@
 
 part of file.src.forwarding;
 
+/// A link that forwards all methods and properties to a delegate.
 abstract class ForwardingLink extends ForwardingFileSystemEntity<Link, io.Link>
     implements Link {
   @override