Have pkg/path track the current working directory if it changes.

pkg/path/lib/path.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.
 
 /// A comprehensive, cross-platform path manipulation library.
 ///
 /// ## Installing ##
 ///
 /// Use [pub][] to install this package. Add the following to your
 /// `pubspec.yaml` file.
@@ skipping 28 matching lines @@
 /// underlying platform that the program is running on, you can create a
 /// [Builder] and give it an explicit [Style]:
 ///
 ///     var builder = new path.Builder(style: Style.windows);
 ///     builder.join("directory", "file.txt");
 ///
 /// This will join "directory" and "file.txt" using the Windows path separator,
 /// even when the program is run on a POSIX machine.
 library path;
 
-/// An internal builder for the current OS so we can provide a straight
-/// functional interface and not require users to create one.
-final _builder = new Builder();
-
 /// A default builder for manipulating POSIX paths.
 final posix = new Builder(style: Style.posix);
 
 /// A default builder for manipulating Windows paths.
 final windows = new Builder(style: Style.windows);
 
 /// A default builder for manipulating URLs.
 final url = new Builder(style: Style.url);
 
 /// Inserts [length] elements in front of the [list] and fills them with the
 /// [fillValue].
 void _growListFront(List list, int length, fillValue) =>
   list.insertAll(0, new List.filled(length, fillValue));
 
+/// The result of [Uri.base] last time the current working directory was
+/// calculated.
+///
+/// This is used to invalidate [_cachedBuilder] when the working directory has
+/// changed since the last time a function was called.
+Uri _lastBaseUri;
+
+/// An internal builder for the current OS so we can provide a straight
+/// functional interface and not require users to create one.
+Builder get _builder {
+  if (_cachedBuilder != null && Uri.base == _lastBaseUri) return _cachedBuilder;
+  _lastBaseUri = Uri.base;
+  _cachedBuilder = new Builder();
+  return _cachedBuilder;
+}
+Builder _cachedBuilder;
 
 /// Gets the path to the current working directory.
 ///
 /// In the browser, this means the current URL, without the last file segment.
 String get current {
   var uri = Uri.base;
   if (Style.platform == Style.url) {
     return uri.resolve('.').toString();
   } else {
     var path = uri.toFilePath();
@@ skipping 1102 matching lines @@
     // doesn't count.
     if (lastDot <= 0) return [file, ''];
 
     return [file.substring(0, lastDot), file.substring(lastDot)];
   }
 
   _ParsedPath clone() => new _ParsedPath(
       style, root, isRootRelative,
       new List.from(parts), new List.from(separators));
 }