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

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

Index: pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart b/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
index bb3ceb2c87cc15ffa1adcdb05481000a043f5159..901b29c4ee2516f1b5c981cec6cb09c28807e92e 100644
--- a/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
@@ -10,6 +10,9 @@ import 'builder.dart' show
     ModifierBuilder,
     TypeBuilder;
 
+import '../util/relativize.dart' show
+    relativizeUri;
+
 abstract class TypeDeclarationBuilder<T extends TypeBuilder, R>
     extends ModifierBuilder {
   final List<MetadataBuilder> metadata;
@@ -20,9 +23,16 @@ abstract class TypeDeclarationBuilder<T extends TypeBuilder, R>
 
   Builder parent;
 
+  final Uri fileUri;
+  final String relativeFileUri;
+
   TypeDeclarationBuilder(this.metadata, this.modifiers, this.name,
       Builder parent, int charOffset, [Uri fileUri])
-      : parent = parent, super(parent, charOffset, fileUri ?? parent?.fileUri);
+    : fileUri = fileUri ?? parent?.fileUri,
+      relativeFileUri = fileUri != null
+                        ? relativizeUri(fileUri)
+                        : parent?.relativeFileUri,
+      super(parent, charOffset, fileUri ?? parent?.fileUri);
 
   bool get isTypeDeclaration => true;