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";

[ahe, 2017/02/21 14:15:23]

I prefer the existing style, and I know dart2js generates better code for it.

-  return result.startsWith("$base") ? result.substring("$base".length) : result;
+  String result = uri.toString();
+  final prefix = base.toString();

[ahe, 2017/02/21 14:15:22]

String prefix = "$base";

[kustermann, 2017/02/22 14:40:28]

I changed it because I saw some interpolate instructions in the flow graph,
which is unnecesary work. Maybe it was just in unoptimized code.

Done.

[ahe, 2017/02/22 14:50:06]

Let's not forget this. I'm pretty sure we have a benchmark that checks if string
interpolation is fast, but it may only be optimized on dart2js.

+  return result.startsWith(prefix) ? result.substring(prefix.length) : result;
 }