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

pkg/front_end/lib/src/fasta/builder/library_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index bf2eb9202d3211d5dce19bb77c969ca4b5559065..61ec1a2ede56f50325cc42210db5f2675055182e 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -17,6 +17,9 @@ import '../export.dart' show
 import '../loader.dart' show
     Loader;
 
+import '../util/relativize.dart' show
+    relativizeUri;
+
 import 'builder.dart' show
     Builder,
     ClassBuilder,
@@ -36,7 +39,8 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
 
   Uri get uri;
 
-  Uri get fileUri;
+  final Uri fileUri;
+  final String relativeFileUri;
 
   Map<String, Builder> get members;
 
@@ -46,7 +50,9 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
   Map<String, Builder> get exports;
 
   LibraryBuilder(Uri fileUri)
-      : super(null, -1, fileUri);
+      : fileUri = fileUri,
+        relativeFileUri = relativizeUri(fileUri),
+        super(null, -1, fileUri);
 
   Builder addBuilder(String name, Builder builder, int charOffset);