pkg/test: calculate the SDK dir in one place

lib/src/util/io.dart

 // Copyright (c) 2015, 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 test.util.io;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:mirrors';
 
 import 'package:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 
 import '../backend/operating_system.dart';
 import '../runner/application_exception.dart';
 
 /// The root directory of the Dart SDK.
 final String sdkDir =
     p.dirname(p.dirname(Platform.executable));
 
 /// The version of the Dart SDK currently in use.
 final Version _sdkVersion = new Version.parse(
-    new File(p.join(p.dirname(p.dirname(Platform.executable)), 'version'))
+    new File(p.join(sdkDir, 'version'))
         .readAsStringSync().trim());
 
 /// Returns the current operating system.
 final OperatingSystem currentOS = (() {
   var name = Platform.operatingSystem;
   var os = OperatingSystem.findByIoName(name);
   if (os != null) return os;
 
   throw new UnsupportedError('Unsupported operating system "$name".');
 })();
@@ skipping 98 matching lines @@
 /// returned.
 String libraryPath(Symbol libraryName, {String packageRoot}) {
   var lib = currentMirrorSystem().findLibrary(libraryName);
   if (lib.uri.scheme != 'package') return p.fromUri(lib.uri);
 
   // TODO(nweiz): is there a way to avoid assuming this is being run next to a
   // packages directory?.
   if (packageRoot == null) packageRoot = p.absolute('packages');
   return p.join(packageRoot, p.fromUri(lib.uri.path));
 }