Don't cache path Context based on cwd, as cwd involves a system-call to compute.

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 31 matching lines @@
 ///     var context = new path.Context(style: Style.windows);
 ///     context.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;
 
 import 'src/context.dart';
 import 'src/style.dart';
 
-export 'src/context.dart';
+export 'src/context.dart' hide createInternal;
 export 'src/path_exception.dart';
 export 'src/style.dart';
 
 /// A default context for manipulating POSIX paths.
 final posix = new Context(style: Style.posix);
 
 /// A default context for manipulating Windows paths.
 final windows = new Context(style: Style.windows);
 
 /// A default context for manipulating URLs.
 final url = new Context(style: Style.url);
 
-/// The result of [Uri.base] last time the current working directory was
-/// calculated.
-///
-/// This is used to invalidate [_cachedContext] when the working directory has
-/// changed since the last time a function was called.
-Uri _lastBaseUri;
-
 /// An internal context for the current OS so we can provide a straight
 /// functional interface and not require users to create one.
-Context get _context {
-  if (_cachedContext != null && Uri.base == _lastBaseUri) return _cachedContext;
-  _lastBaseUri = Uri.base;
-  _cachedContext = new Context();
-  return _cachedContext;
-}
-Context _cachedContext;
+final Context _context = createInternal();
 
 /// Returns the [Style] of the current context.
 ///
 /// This is the style that all top-level path functions will use.
 Style get style => _context.style;
 
 /// Gets the path to the current working directory.
 ///
 /// In the browser, this means the current URL, without the last file segment.
 String get current {
@@ skipping 291 matching lines @@
 ///
 ///     // Windows at "C:\root\path"
 ///     path.prettyUri('file:///C:/root/path/a/b.dart'); // -> r'a\b.dart'
 ///     path.prettyUri('http://dartlang.org/'); // -> 'http://dartlang.org'
 ///
 ///     // URL at "http://dartlang.org/root/path"
 ///     path.prettyUri('http://dartlang.org/root/path/a/b.dart');
 ///         // -> r'a/b.dart'
 ///     path.prettyUri('file:///root/path'); // -> 'file:///root/path'
 String prettyUri(uri) => _context.prettyUri(uri);