Don't use Uri's isAbsolute to detect if the path segment is absolute, in toFilePath.

sdk/lib/core/uri.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.core;
 
 /**
  * A parsed URI, as specified by RFC-3986, http://tools.ietf.org/html/rfc3986.
  */
 class Uri {
@@ skipping 953 matching lines @@
   }
 
   String _toFilePath() {
     if (host != "") {
       throw new UnsupportedError(
           "Cannot extract a non-Windows file path from a file URI "
           "with an authority");
     }
     _checkNonWindowsPathReservedCharacters(pathSegments, false);
     var result = new StringBuffer();
-    if (isAbsolute) result.write("/");
+    if (_isPathAbsolute) result.write("/");
     result.writeAll(pathSegments, "/");
     return result.toString();
   }
 
   String _toWindowsFilePath() {
     bool hasDriveLetter = false;
     var segments = pathSegments;
     if (segments.length > 0 &&
         segments[0].length == 2 &&
         segments[0].codeUnitAt(1) == _COLON) {
       _checkWindowsDriveLetter(segments[0].codeUnitAt(0), false);
       _checkWindowsPathReservedCharacters(segments, false, 1);
       hasDriveLetter = true;
     } else {
       _checkWindowsPathReservedCharacters(segments, false);
     }
     var result = new StringBuffer();
-    if (isAbsolute && !hasDriveLetter) result.write("\\");
+    if (_isPathAbsolute && !hasDriveLetter) result.write("\\");
     if (host != "") {
       result.write("\\");
       result.write(host);
       result.write("\\");
     }
     result.writeAll(segments, "\\");
     if (hasDriveLetter && segments.length == 1) result.write("\\");
     return result.toString();
   }
 
+  bool get _isPathAbsolute {
+    if (path == null || path.isEmpty) return false;
+    return path.startsWith('/');
+  }
+
   void _writeAuthority(StringSink ss) {
     _addIfNonEmpty(ss, userInfo, userInfo, "@");
     ss.write(_host == null ? "null" : _host);
     if (port != 0) {
       ss.write(":");
       ss.write(port.toString());
     }
   }
 
   String toString() {
@@ skipping 622 matching lines @@
   void clear() {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
   void forEach(void f(K key, V value)) => _map.forEach(f);
   Iterable<K> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
 }