Support browser contexts in package:file (flutter#18)

This change uses conditional imports to support browser-based
Dart usage of this package. For non-VM users, this will export
the subset of the dart:io interface that is required by this
package, whereas VM users will simply get dart:io itself.

Author: tvolkert

.analysis_options

@@ -1,2 +1,3 @@
 analyzer:
   strong-mode: true
+enableConditionalDirectives: true

lib/file.dart

@@ -1,5 +1 @@
-export 'src/interface/directory.dart';
-export 'src/interface/file_system_entity.dart';
-export 'src/interface/file_system.dart';
-export 'src/interface/file.dart';
-export 'src/interface/link.dart';
+export 'src/interface.dart';

lib/src/backends/local.dart

@@ -2,10 +2,12 @@ library file.src.backends.local;
 
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io' as io;
 
+import 'package:file/src/io.dart' as io;
 import 'package:file/file.dart';
 
+import '../io/shim.dart' as shim;
+
 part 'local/local_directory.dart';
 part 'local/local_file.dart';
 part 'local/local_file_system.dart';

lib/src/backends/local/local_file_system.dart

@@ -9,42 +9,42 @@ class LocalFileSystem extends FileSystem {
 
   @override
   Directory directory(String path) =>
-      new _LocalDirectory(this, new io.Directory(path));
+      new _LocalDirectory(this, shim.newDirectory(path));
 
   @override
-  File file(String path) => new _LocalFile(this, new io.File(path));
+  File file(String path) => new _LocalFile(this, shim.newFile(path));
 
   @override
-  Link link(String path) => new _LocalLink(this, new io.Link(path));
+  Link link(String path) => new _LocalLink(this, shim.newLink(path));
 
   @override
-  Directory get currentDirectory => directory(io.Directory.current.path);
+  Directory get currentDirectory => directory(shim.currentDirectory.path);
 
   @override
-  set currentDirectory(dynamic path) => io.Directory.current = path;
+  set currentDirectory(dynamic path) => shim.currentDirectory = path;
 
   @override
-  Future<io.FileStat> stat(String path) => io.FileStat.stat(path);
+  Future<io.FileStat> stat(String path) => shim.stat(path);
 
   @override
-  io.FileStat statSync(String path) => io.FileStat.statSync(path);
+  io.FileStat statSync(String path) => shim.statSync(path);
 
   @override
   Future<bool> identical(String path1, String path2) =>
-      io.FileSystemEntity.identical(path1, path2);
+      shim.identical(path1, path2);
 
   @override
   bool identicalSync(String path1, String path2) =>
-      io.FileSystemEntity.identicalSync(path1, path2);
+      shim.identicalSync(path1, path2);
 
   @override
-  bool get isWatchSupported => io.FileSystemEntity.isWatchSupported;
+  bool get isWatchSupported => shim.isWatchSupported;
 
   @override
   Future<io.FileSystemEntityType> type(String path, {bool followLinks: true}) =>
-      io.FileSystemEntity.type(path, followLinks: followLinks);
+      shim.type(path, followLinks);
 
   @override
   io.FileSystemEntityType typeSync(String path, {bool followLinks: true}) =>
-      io.FileSystemEntity.typeSync(path, followLinks: followLinks);
+      shim.typeSync(path, followLinks);
 }

lib/src/backends/memory.dart

@@ -2,9 +2,9 @@ library file.src.backends.memory;
 
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io' as io;
 
 import 'package:file/file.dart';
+import 'package:file/src/io.dart' as io;
 import 'package:path/path.dart' as p;
 
 part 'memory/memory_directory.dart';

lib/src/interface.dart

@@ -0,0 +1,13 @@
+library file.src.interface;
+
+import 'dart:async';
+
+import 'io.dart' as io;
+
+export 'io.dart';
+
+part 'interface/directory.dart';
+part 'interface/file.dart';
+part 'interface/file_system.dart';
+part 'interface/file_system_entity.dart';
+part 'interface/link.dart';

lib/src/interface/directory.dart

@@ -1,6 +1,4 @@
-import 'dart:io' as io;
-
-import 'file_system_entity.dart';
+part of file.src.interface;
 
 /// A reference to a directory on the file system.
 abstract class Directory implements FileSystemEntity, io.Directory {}

lib/src/interface/file.dart

@@ -1,6 +1,4 @@
-import 'dart:io' as io;
-
-import 'file_system_entity.dart';
+part of file.src.interface;
 
 /// A reference to a file on the file system.
 abstract class File implements FileSystemEntity, io.File {}

lib/src/interface/file_system.dart

@@ -1,9 +1,4 @@
-import 'dart:async';
-import 'dart:io' as io;
-
-import 'directory.dart';
-import 'file.dart';
-import 'link.dart';
+part of file.src.interface;
 
 /// A generic representation of a file system.
 ///

lib/src/interface/file_system_entity.dart

@@ -1,6 +1,4 @@
-import 'dart:io' as io;
-
-import 'file_system.dart';
+part of file.src.interface;
 
 /// The common super class for [File], [Directory], and [Link] objects.
 abstract class FileSystemEntity implements io.FileSystemEntity {