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..e3fc03af6edf3579a0f77cd57780474fc9e7239b 100644
--- a/pkg/front_end/lib/src/fasta/util/relativize.dart
+++ b/pkg/front_end/lib/src/fasta/util/relativize.dart
@@ -10,5 +10,6 @@ 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;
 }