dart:io | The empty path should return "." for its toNativePath value.

sdk/lib/io/path_impl.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.
 
 part of dart.io;
 
 class _Path implements Path {
   final String _path;
   final bool isWindowsShare;
 
@@ skipping 136 matching lines @@
       return further.canonicalize();
     }
     if (hasTrailingSeparator) {
       var joined = new _Path._internal('$_path${further}', isWindowsShare);
       return joined.canonicalize();
     }
     var joined = new _Path._internal('$_path/${further}', isWindowsShare);
     return joined.canonicalize();
   }
 
-  // Note: The URI RFC names for these operations are normalize, resolve, and
-  // relativize.
+  // Note: The URI RFC names for canonicalize, join, and relativeTo
+  // are normalize, resolve, and relativize.  But resolve and relativize
+  // drop the last segment of the base path (the filename), on URIs.
   Path canonicalize() {
     if (isCanonical) return this;
     return makeCanonical();
   }
 
   bool get isCanonical {
     // Contains no consecutive path separators.
     // Contains no segments that are '.'.
     // Absolute paths have no segments that are '..'.
     // All '..' segments of a relative path are at the beginning.
@@ skipping 69 matching lines @@
     } else {
       segmentsToJoin.addAll(newSegs);
       if (hasTrailingSeparator) {
         segmentsToJoin.add('');
       }
     }
     return new _Path._internal(segmentsToJoin.join('/'), isWindowsShare);
   }
 
   String toNativePath() {
+    if (isEmpty) return '.';
     if (Platform.operatingSystem == 'windows') {
       String nativePath = _path;
       // Drop '/' before a drive letter.
       if (nativePath.length >= 3 &&
           nativePath.startsWith('/') &&
           nativePath[2] == ':') {
         nativePath = nativePath.substring(1);
       }
       nativePath = nativePath.replaceAll('/', '\\');
       if (isWindowsShare) {
@@ skipping 40 matching lines @@
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     var dirPath = (pos > 0) ? _path.substring(0, pos) : '/';
     return new _Path._internal(dirPath, isWindowsShare);
   }
 
   String get filename {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }