Re-land "Add support for metadata on type variables to Fasta parser." (with fixes).

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 3c8b5f09d2c7b3e2202918d317b306b08819b226..073674bb5f326b2b8a45f607d98ea3145950792f 100644
--- a/pkg/front_end/lib/src/fasta/parser/listener.dart
+++ b/pkg/front_end/lib/src/fasta/parser/listener.dart
@@ -537,6 +537,11 @@ 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");
   }
@@ -728,6 +733,7 @@ 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) {