Ensure locations are always provided, but don't store them yet.

pkg/front_end/lib/src/fasta/util/relativize.dart

+// Copyright (c) 2017, 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 fasta.util.relativize;
+
+// TODO(ahe): Move more advanced version from dart2js here.
+
+String relativizeUri(Uri uri, {Uri base}) {
+  if (uri == null) return null;
+  base ??= Uri.base;
+  String result = "$uri";
+  return result.startsWith("$base") ? result.substring("$base".length) : result;
+}