Get rid of join() and encapsulate File and Directory in io.dart.

utils/pub/sdk.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.
 
 /// Operations relative to the user's installed Dart SDK.
 library sdk;
 
 import 'dart:io';
 
 import '../../pkg/path/lib/path.dart' as path;
+
+import 'io.dart';
 import 'log.dart' as log;
 import 'version.dart';
 
 /// Matches an Eclipse-style SDK version number. This is four dotted numbers
 /// (major, minor, patch, build) with an optional suffix attached to the build
 /// number.
 final _versionPattern = new RegExp(r'^(\d+)\.(\d+)\.(\d+)\.(\d+.*)$');
 
 /// Gets the path to the root directory of the SDK.
 String get rootDirectory {
@@ skipping 10 matching lines @@
   log.fine("Located SDK at $dir");
   return dir;
 }
 
 /// Gets the SDK's revision number formatted to be a semantic version.
 Version version = _getVersion();
 
 /// Determine the SDK's version number.
 Version _getVersion() {
   var revisionPath = path.join(rootDirectory, "version");
-  var version = new File(revisionPath).readAsStringSync().trim();
+  var version = readTextFile(revisionPath).trim();
 
   // Given a version file like: 0.1.2.0_r17495
   // We create a semver like:   0.1.2+0.r17495
   var match = _versionPattern.firstMatch(version);
   if (match == null) {
     throw new FormatException("The Dart SDK's 'version' file was not in a "
         "format pub could recognize. Found: $version");
   }
 
   // Semantic versions cannot use "_".
   var build = match[4].replaceAll('_', '.');
 
   return new Version(
       int.parse(match[1]), int.parse(match[2]), int.parse(match[3]),
       build: build);
 }