fasta: Cache relativizeUri(fileUri) on LibraryBuilder and ClassBuilder (or rather it's base class)

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("$base".length) : result;
+  final prefix = "$base";
+  return result.startsWith(prefix) ? result.substring(prefix.length) : result;
 }