Add AstBuilder support for fields.

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

Index: pkg/compiler/lib/src/parser/member_listener.dart
diff --git a/pkg/compiler/lib/src/parser/member_listener.dart b/pkg/compiler/lib/src/parser/member_listener.dart
index 4697d5b2de7d05d1bf796bd37719725fb954af44..b1684cb1453481ea7859980a5df723ad5af8a664 100644
--- a/pkg/compiler/lib/src/parser/member_listener.dart
+++ b/pkg/compiler/lib/src/parser/member_listener.dart
@@ -115,9 +115,10 @@ class MemberListener extends NodeListener {
   }
 
   @override
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantKeyword, Token beginToken, Token endToken) {
     bool hasParseError = memberErrors.head;
-    super.endFields(count, beginToken, endToken);
+    super.endFields(count, covariantKeyword, beginToken, endToken);
     VariableDefinitions variableDefinitions = popNode();
     Modifiers modifiers = variableDefinitions.modifiers;
     pushNode(null);