Add AstBuilder support for fields.

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

Index: pkg/front_end/lib/src/fasta/source/diet_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/diet_listener.dart b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
index ac7e74f78de0824c22ddf4d6fb897aff07869295..8f8ce4f822634790fca3742f460581377e5b06a9 100644
--- a/pkg/front_end/lib/src/fasta/source/diet_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
@@ -178,7 +178,8 @@ class DietListener extends StackListener {
   }
 
   @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);
     Builder builder = lookupBuilder(beginToken, null, names.first);