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

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

 // Copyright (c) 2016, 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.library_builder;
 
 import '../combinator.dart' show
     Combinator;
 
 import '../errors.dart' show
     InputError,
     internalError;
 
 import '../export.dart' show
     Export;
 
 import '../loader.dart' show
     Loader;
 
+import '../util/relativize.dart' show
+    relativizeUri;
+
 import 'builder.dart' show
     Builder,
     ClassBuilder,
     TypeBuilder;
 
 import 'scope.dart' show
    Scope;
 
 abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
   final List<Export> exporters = <Export>[];
 
   final List<InputError> compileTimeErrors = <InputError>[];
 
   LibraryBuilder partOfLibrary;
 
   Loader get loader;
 
   Uri get uri;
 
-  Uri get fileUri;
+  final Uri fileUri;
+  final String relativeFileUri;
 
   Map<String, Builder> get members;
 
   // TODO(ahe): Move this to SourceLibraryBuilder.
   Scope get scope;
 
   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);
 
   void addExporter(LibraryBuilder exporter, List<Combinator> combinators,
       int charOffset) {
     exporters.add(new Export(exporter, this, combinators, charOffset));
   }
 
   void addCompileTimeError(int charOffset, Object message, [Uri fileUri]) {
     fileUri ??= this.fileUri;
@@ skipping 36 matching lines @@
       } else if (constructor.isFactory) {
         return constructor;
       }
     }
     throw internalError("Internal error: No constructor named"
         " '$className::$constructorName' in '$uri'.");
   }
 
   int finishTypeVariables(ClassBuilder object) => 0;
 }