Support root-relative paths on Windows.

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 542 matching lines @@
   /// For a fixed number of parts, [join] is usually terser.
   String joinAll(Iterable<String> parts) {
     var buffer = new StringBuffer();
     var needsSeparator = false;
     var isAbsoluteAndNotRootRelative = false;
 
     for (var part in parts.where((part) => part != '')) {
       if (this.isRootRelative(part) && isAbsoluteAndNotRootRelative) {
         // If the new part is root-relative, it preserves the previous root but
         // replaces the path after it.
-        var oldRoot = this.rootPrefix(buffer.toString());
+        var parsed = _parse(part);
+        parsed.root = this.rootPrefix(buffer.toString());
+        if (parsed.root.contains(style.needsSeparatorPattern)) {
+          parsed.separators[0] = style.separator;
+        }
         buffer.clear();
-        buffer.write(oldRoot);
-        buffer.write(part);
+        buffer.write(parsed);
       } else if (this.isAbsolute(part)) {
         isAbsoluteAndNotRootRelative = !this.isRootRelative(part);
         // An absolute path discards everything before it.
         buffer.clear();
         buffer.write(part);
       } else {
         if (part.length > 0 && part[0].contains(style.separatorPattern)) {
           // The part starts with a separator, so we don't need to add one.
         } else if (needsSeparator) {
           buffer.write(separator);
@@ skipping 399 matching lines @@
 /// The style for Windows paths.
 class _WindowsStyle extends Style {
   _WindowsStyle();
 
   final name = 'windows';
   final separator = '\\';
   final separatorPattern = new RegExp(r'[/\\]');
   final needsSeparatorPattern = new RegExp(r'[^/\\]$');
   final rootPattern = new RegExp(r'^(\\\\[^\\]+\\[^\\/]+|[a-zA-Z]:[/\\])');
 
+  // Matches a back or forward slash that's not followed by another back or
+  // forward slash.
+  final relativeRootPattern = new RegExp(r"^[/\\](?![/\\])");
+
   String pathFromUri(Uri uri) {
     if (uri.scheme != '' && uri.scheme != 'file') {
       throw new ArgumentError("Uri $uri must have scheme 'file:'.");
     }
 
     var path = uri.path;
     if (uri.host == '') {
       // Drive-letter paths look like "file:///C:/path/to/file". The
       // replaceFirst removes the extra initial slash.
       if (path.startsWith('/')) path = path.replaceFirst("/", "");
@@ skipping 194 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));
 }