Add AstBuilder support for fields.

pkg/compiler/lib/src/parser/node_listener.dart

Index: pkg/compiler/lib/src/parser/node_listener.dart
diff --git a/pkg/compiler/lib/src/parser/node_listener.dart b/pkg/compiler/lib/src/parser/node_listener.dart
index 4317efc73b78099fd7798760d44a6353c9e8e3da..2a91331d188faed5c92244aa52f8f9d75a171927 100644
--- a/pkg/compiler/lib/src/parser/node_listener.dart
+++ b/pkg/compiler/lib/src/parser/node_listener.dart
@@ -664,7 +664,8 @@ class NodeListener extends ElementListener {
   }
 
   @override
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantKeyword, Token beginToken, Token endToken) {
     NodeList variables = makeNodeList(count, null, endToken, ",");
     TypeAnnotation type = popNode();
     Modifiers modifiers = popNode();