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

pkg/path/lib/src/context.dart

 // Copyright (c) 2013, 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 path.context;
 
 import 'internal_style.dart';
 import 'style.dart';
 import 'parsed_path.dart';
 import 'path_exception.dart';
 import '../path.dart' as p;
 
+Context createInternal() => new Context._internal();
+
 /// An instantiable class for manipulating paths. Unlike the top-level
 /// functions, this lets you explicitly select what platform the paths will use.
 class Context {
   /// Creates a new path context for the given style and current directory.
   ///
   /// If [style] is omitted, it uses the host operating system's path style. If
   /// only [current] is omitted, it defaults ".". If *both* [style] and
   /// [current] are omitted, [current] defaults to the real current working
   /// directory.
   ///
@@ skipping 11 matching lines @@
     if (style == null) {
       style = Style.platform;
     } else if (style is! InternalStyle) {
       throw new ArgumentError("Only styles defined by the path package are "
           "allowed.");
     }
 
     return new Context._(style, current);
   }
 
-  Context._(this.style, this.current);
+  /// Create a [Context] to be used internally within path.
+  Context._internal() : style = Style.platform;
+
+  Context._(this.style, this._current);
 
   /// The style of path that this context works with.
   final InternalStyle style;
 
-  /// The current directory that relative paths will be relative to.
-  final String current;
+  /// The current directory given when Context was created. If null, current
+  /// directory is evaluated from 'p.current'.
+  final String _current;
+
+  /// The current directory that relative paths are relative to.
+  String get current => _current != null ? _current : p.current;
 
   /// Gets the path separator for the context's [style]. On Mac and Linux,
   /// this is `/`. On Windows, it's `\`.
   String get separator => style.separator;
 
   /// Creates a new path by appending the given path parts to [current].
   /// Equivalent to [join()] with [current] as the first argument. Example:
   ///
   ///     var context = new Context(current: '/root');
   ///     context.absolute('path', 'to', 'foo'); // -> '/root/path/to/foo'
@@ skipping 477 matching lines @@
     // Show the arguments.
     var message = new StringBuffer();
     message.write("$method(");
     message.write(args.take(numArgs)
         .map((arg) => arg == null ? "null" : '"$arg"')
         .join(", "));
     message.write("): part ${i - 1} was null, but part $i was not.");
     throw new ArgumentError(message.toString());
   }
 }