Change NotImplementedException to UnimplementedError.

lib/io/path_impl.dart

Index: lib/io/path_impl.dart
diff --git a/lib/io/path_impl.dart b/lib/io/path_impl.dart
index d3467dfacd05c25a36abb30ee307dc6e4ca494e5..8e05d447da84ec9ac2f33d69d771b389af2d165d 100644
--- a/lib/io/path_impl.dart
+++ b/lib/io/path_impl.dart
@@ -75,7 +75,7 @@ class _Path implements Path {
       }
       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)");