Fix internal constructor in context.

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';
@@ skipping 26 matching lines @@
       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);
   }
 
   /// Create a [Context] to be used internally within path.
-  Context._internal() : style = Style.platform;
+  Context._internal() : style = Style.platform, _current = null;
 
   Context._(this.style, this._current);
 
   /// The style of path that this context works with.
   final InternalStyle style;
 
   /// The current directory given when Context was created. If null, current
   /// directory is evaluated from 'p.current'.
   final String _current;
 
@@ skipping 489 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());
   }
 }