Handle TypeVariable and TypeVariables in AstBuilder.

pkg/front_end/lib/src/fasta/parser/listener.dart

Index: pkg/front_end/lib/src/fasta/parser/listener.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/listener.dart b/pkg/front_end/lib/src/fasta/parser/listener.dart
index 95dfdae23067f7b3bd03812de26b507aac1af254..6100785b2b196c036231b78d5da75c53a2d9c70b 100644
--- a/pkg/front_end/lib/src/fasta/parser/listener.dart
+++ b/pkg/front_end/lib/src/fasta/parser/listener.dart
@@ -628,6 +628,10 @@ class Listener {
 
   void beginTypeVariable(Token token) {}
 
+  /// Handle the end of a type formal parameter (e.g. "X extends Y").
+  /// Substructures:
+  /// - Name (identifier)
+  /// - Type bound
   void endTypeVariable(Token token, Token extendsOrSuper) {
     logEvent("TypeVariable");
   }