Add AstBuilder support for fields.

pkg/front_end/lib/src/fasta/parser/parser.dart

Index: pkg/front_end/lib/src/fasta/parser/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/parser.dart b/pkg/front_end/lib/src/fasta/parser/parser.dart
index c517e22bcac260aa776657a12b64d5bba3bfc589..fd1f9457a15f449f87d2b99e3565c40667bd9a87 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -1165,8 +1165,13 @@ class Parser {
       Token getOrSet, Token name, bool isTopLevel) {
     bool hasType = type != null;
 
+    Token covariantKeyword;
     if (getOrSet == null && !isTopLevel) {
-      modifiers = removeOptCovariantTokenIfNotStatic(modifiers);
+      Link<Token> newModifiers = removeOptCovariantTokenIfNotStatic(modifiers);

[ahe, 2017/03/01 15:20:31]

This is horrible. But that's not your fault. Would you mind adding a todo for me
to completely get rid of the method removeOptCovariantTokenIfNotStatic?

[Paul Berry, 2017/03/01 21:08:22]

Done.

+      if (!identical(newModifiers, modifiers)) {
+        covariantKeyword = modifiers.first;
+        modifiers = newModifiers;
+      }
     }
 
     Token varFinalOrConst =
@@ -1219,7 +1224,7 @@ class Parser {
     if (isTopLevel) {
       listener.endTopLevelFields(fieldCount, start, semicolon);
     } else {
-      listener.endFields(fieldCount, start, semicolon);
+      listener.endFields(fieldCount, covariantKeyword, start, semicolon);
     }
     return token;
   }
@@ -1716,7 +1721,7 @@ class Parser {
         token = reportUnrecoverableError(token, ErrorKind.UnexpectedToken);
         if (identical(token.kind, EOF_TOKEN)) {
           // TODO(ahe): This is a hack, see parseTopLevelMember.
-          listener.endFields(1, start, token);
+          listener.endFields(1, null, start, token);
           listener.endMember();
           return token;
         }