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

pkg/front_end/lib/src/fasta/builder/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.builder;
 
 import '../errors.dart' show
     internalError;
 
 import '../messages.dart' show
@@ skipping 81 matching lines @@
 
   /// The values of [parent], [charOffset], and [fileUri] aren't stored. We
   /// need to evaluate the memory impact of doing so, but want to ensure the
   /// information is always provided.
   Builder(Builder parent, int charOffset, Uri fileUri);
 
   int get charOffset => -1;
 
   Uri get fileUri => null;
 
+  String get relativeFileUri {
+    throw "The relativeFileUri method should be only called on subclasses "
+          "which have an efficient implementation of `relativeFileUri`!";
+  }
+
   /// Resolve types (lookup names in scope) recorded in this builder and return
   /// the number of types resolved.
   int resolveTypes(covariant Builder parent) => 0;
 
   /// Resolve constructors (lookup names in scope) recorded in this builder and
   /// return the number of constructors resolved.
   int resolveConstructors(covariant Builder parent) => 0;
 
   /// This builder and [other] has been imported into [library] using [name].
   ///
@@ skipping 65 matching lines @@
 
   static Uri getUri(Builder builder) {
     if (builder == null) return internalError("Builder is null.");
     while (builder != null) {
       if (builder is LibraryBuilder) return builder.uri;
       builder = builder.parent;
     }
     return internalError("No library parent.");
   }
 }