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

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

Index: pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
index fcc7bd58161686f211e8717a4f3cfe1eee283041..9cce81d4e2e9f7a4a684093c30eb3c336b225483 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -1387,9 +1387,8 @@ class AstBuilder extends ScopeListener {
     debugEvent("TypeVariable");
     TypeAnnotation bound = pop();
     SimpleIdentifier name = pop();
-    List<Annotation> metadata = null; // TODO(paulberry)
-    // TODO(paulberry): capture doc comments.  See dartbug.com/28851.
-    Comment comment = null;
+    List<Annotation> metadata = pop();
+    Comment comment = pop();
     push(ast.typeParameter(
         comment, metadata, name, toAnalyzerToken(extendsOrSuper), bound));
   }