Add support for metadata on type variables to Fasta parser.

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 a9c1ef6b935b6a365405844db319c40aad546af0..21861bdd41cf854f6f42f8ff0e246d6ac61563d2 100644
--- a/pkg/front_end/lib/src/fasta/parser/listener.dart
+++ b/pkg/front_end/lib/src/fasta/parser/listener.dart
@@ -520,6 +520,11 @@ class Listener {
 
   void beginMetadata(Token token) {}
 
+  /// Handle the end of a metadata annotation.  Substructures:
+  /// - Identifier
+  /// - Type arguments
+  /// - Constructor name (only if [periodBeforeName] is not `null`)
+  /// - Arguments
   void endMetadata(Token beginToken, Token periodBeforeName, Token endToken) {
     logEvent("Metadata");
   }
@@ -704,6 +709,7 @@ class Listener {
 
   /// Handle the end of a type formal parameter (e.g. "X extends Y").
   /// Substructures:
+  /// - Metadata
   /// - Name (identifier)
   /// - Type bound
   void endTypeVariable(Token token, Token extendsOrSuper) {