Handle type variables.

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

Index: pkg/front_end/lib/src/fasta/builder/interface_type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/interface_type_builder.dart b/pkg/front_end/lib/src/fasta/builder/interface_type_builder.dart
index e0d0311590345ea9b937ea1f0e1ea2a5b91eb388..cd34e65336a0a93f61aa75f5c36e6e0d63a5f249 100644
--- a/pkg/front_end/lib/src/fasta/builder/interface_type_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/interface_type_builder.dart
@@ -29,6 +29,10 @@ abstract class InterfaceTypeBuilder<T extends TypeBuilder> extends TypeBuilder {
 
   InvalidTypeBuilder buildInvalidType(String name);
 
+  void bind(TypeDeclarationBuilder builder) {
+    this.builder = builder;
+  }
+
   void resolveIn(Scope scope) {
     Builder member = scope.lookup(name);
     if (member is TypeDeclarationBuilder) {