Change NotImplementedException to UnimplementedError.

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.
 
 class _Path implements Path {
   final String _path;
 
   _Path(String source) : _path = source;
   _Path.fromNative(String source) : _path = _clean(source);
 
@@ skipping 57 matching lines @@
       }
       for (int i = common; i < pathSegments.length - 1; i++) {
         sb.add('${pathSegments[i]}/');
       }
       sb.add('${pathSegments.last}');
       if (hasTrailingSeparator) {
         sb.add('/');
       }
       return new Path(sb.toString());
     }
-    throw new NotImplementedException(
+    throw new UnimplementedError(
       "Unimplemented case of Path.relativeTo(base):\n"
       "  Only absolute paths are handled at present.\n"
       "  Arguments: $_path.relativeTo($base)");
   }
 
   Path join(Path further) {
     if (further.isAbsolute) {
       throw new ArgumentError(
           "Path.join called with absolute Path as argument.");
     }
@@ skipping 146 matching lines @@
     if (pos < 0) return new Path('');
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     return new Path((pos > 0) ? _path.substring(0, pos) : '/');
   }
 
   String get filename {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }