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

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

Index: pkg/front_end/lib/src/fasta/util/relativize.dart
diff --git a/pkg/front_end/lib/src/fasta/util/relativize.dart b/pkg/front_end/lib/src/fasta/util/relativize.dart
index 59e9f16b290ed986f35b02698718b58fc6f26862..bae65ede779f3c82e00161bee48f8858885124e0 100644
--- a/pkg/front_end/lib/src/fasta/util/relativize.dart
+++ b/pkg/front_end/lib/src/fasta/util/relativize.dart
@@ -9,6 +9,7 @@ library fasta.util.relativize;
 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;
 }