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

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

Index: pkg/front_end/lib/src/fasta/builder/builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/builder.dart b/pkg/front_end/lib/src/fasta/builder/builder.dart
index f1ed1b4de366c1815f9e5e430c838af6cdca1c26..89e5f970fa793aaacadc76106d6f47e76956a860 100644
--- a/pkg/front_end/lib/src/fasta/builder/builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/builder.dart
@@ -99,6 +99,11 @@ abstract class Builder {
 
   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;