First stab at a dev server in pub using barback.

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 37 matching lines @@
 
 @MirrorsUsed(targets: 'dart.dom.html.window, '
     'dart.io.Directory.current, '
     'dart.io.Platform.operatingSystem')
 import 'dart:mirrors';
 
 /// 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();
 
-/**
- * Inserts [length] elements in front of the [list] and fills them with the
- * [fillValue].
- */
+/// 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));
 
 /// If we're running in the server-side Dart VM, this will return a
 /// [LibraryMirror] that gives access to the `dart:io` library.
 ///
 /// If `dart:io` is not available, this returns null.
 LibraryMirror get _io => currentMirrorSystem().libraries[Uri.parse('dart:io')];
 
 // TODO(nweiz): when issue 6490 or 6943 are fixed, make this work under dart2js.
@@ skipping 297 matching lines @@
     throw new ArgumentError(message.toString());
   }
 }
 
 /// An instantiable class for manipulating paths. Unlike the top-level
 /// functions, this lets you explicitly select what platform the paths will use.
 class Builder {
   /// Creates a new path builder for the given style and root directory.
   ///
   /// If [style] is omitted, it uses the host operating system's path style. If
-  /// [root] is omitted, it defaults to the current working directory. If [root]
-  /// is relative, it is considered relative to the current working directory.
+  /// [root] is omitted, it defaults ".". If *both* [style] and [root] are

[nweiz, 2013/07/29 23:54:44]

"If [root] is omitted" -> "If only [root] is omitted".

[Bob Nystrom, 2013/07/30 01:04:27]

Done.

+  /// omitted, [root] defaults to the current working directory.
   ///
   /// On the browser, the path style is [Style.url]. In Dartium, [root] defaults
   /// to the current URL. When using dart2js, it currently defaults to `.` due
   /// to technical constraints.
   factory Builder({Style style, String root}) {
-    if (style == null) {
-      if (_io == null) {
-        style = Style.url;
-      } else if (_io.classes[const Symbol('Platform')]
-          .getField(const Symbol('operatingSystem')).reflectee == 'windows') {
-        style = Style.windows;
+    if (root == null) {
+      if (style == null) {
+        root = current;
       } else {
-        style = Style.posix;
+        root = ".";
       }
     }
 
-    if (root == null) root = current;
+    if (style == null) style = Style.platform;
 
     return new Builder._(style, root);
   }
 
   Builder._(this.style, this.root);
 
   /// The style of path that this builder works with.
   final Style style;
 
   /// The root directory that relative paths will be relative to.
@@ skipping 440 matching lines @@
   static final windows = new _WindowsStyle();
 
   /// URLs aren't filesystem paths, but they're supported to make it easier to
   /// manipulate URL paths in the browser.
   ///
   /// URLs use "/" (forward slash) as separators. Absolute paths either start
   /// with a protocol and optional hostname (e.g. `http://dartlang.org`,
   /// `file://`) or with "/".
   static final url = new _UrlStyle();
 
+  /// The style of the host platform.
+  ///
+  /// When running on the command line, this will be [windows] or [posix] based
+  /// on the host operating system. On a browser, this will be [url].
+  static final platform = _getPlatformStyle();
+
+  /// Gets the type of the host platform.
+  static Style _getPlatformStyle() {
+    if (_io == null) return Style.url;
+
+    if (_io.classes[const Symbol('Platform')]
+        .getField(const Symbol('operatingSystem')).reflectee == 'windows') {
+      return Style.windows;
+    }
+
+    return Style.posix;
+  }
+
   /// The name of this path style. Will be "posix" or "windows".
   String get name;
 
   /// The path separator for this style. On POSIX, this is `/`. On Windows,
   /// it's `\`.
   String get separator;
 
   /// The [Pattern] that can be used to match a separator for a path in this
   /// style. Windows allows both "/" and "\" as path separators even though "\"
   /// is the canonical one.
@@ skipping 308 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));
 }