Revert "Move dart2js from sdk/lib/_internal/compiler to pkg/compiler"

pkg/compiler/lib/src/filenames.dart

-// Copyright (c) 2012, 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.
-
-library filenames;
-
-import 'dart:io' show Platform;
-
-// For information about how to convert Windows file names to URIs:
-// http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
-
-String nativeToUriPath(String filename) {
-  // TODO(ahe): It would be nice to use a Dart library instead.
-  if (!Platform.isWindows) return filename;
-  filename = filename.replaceAll('\\', '/');
-  if (filename.length > 2 && filename[1] == ':') {
-    filename = "/$filename";
-  }
-  return filename;
-}
-
-String uriPathToNative(String path) {
-  // TODO(ahe): It would be nice to use a Dart library instead.
-  if (!Platform.isWindows) return path;
-  if (path.length > 3 && path[0] == '/' && path[2] == ':') {
-    return path.substring(1).replaceAll('/', '\\');
-  } else {
-    return path.replaceAll('/', '\\');
-  }
-}
-
-final Uri currentDirectory = Uri.base;
-
-String appendSlash(String path) => path.endsWith('/') ? path : '$path/';