Add AstBuilder support for fields.

pkg/front_end/lib/src/fasta/source/outline_builder.dart

Index: pkg/front_end/lib/src/fasta/source/outline_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/outline_builder.dart b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
index bd466f018442a715a27584b8d6df4ab5c86120f8..20250b2e2d60b9552d6501840a03809512eddd5b 100644
--- a/pkg/front_end/lib/src/fasta/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
@@ -491,7 +491,8 @@ class OutlineBuilder extends UnhandledListener {
   }
 
   @override
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantToken, Token beginToken, Token endToken) {
     debugEvent("Fields");
     List<String> names = popList(count);
     TypeBuilder type = pop();