Reapply "Tweak public APIs and use them in patch_sdk, dart2js, and kernel-service.""

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 598f5e7d570cf55de3a8bd4d0d8f41a026962324..8ec419c84f7811791d6f067489b7ac9b482313a9 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -47,6 +47,8 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
   /// True if a compile-time error has been reported in this library.
   bool hasCompileTimeErrors = false;
 
+  bool mayImplementRestrictedTypes = false;
+
   LibraryBuilder(Uri fileUri, this.scope, this.exports)
       : fileUri = fileUri,
         relativeFileUri = relativizeUri(fileUri),