Add AstBuilder support for fields.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.parser.parser;
 
 import '../scanner.dart' show ErrorToken;
 
 import '../scanner/recover.dart' show closeBraceFor, skipToEof;
 
@@ skipping 1147 matching lines @@
         return modifiers;
       }
     }
     return modifiers.tail;
   }
 
   Token parseFields(Token start, Link<Token> modifiers, Token type,
       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 =
         expectVarFinalOrConst(modifiers, hasType, !isTopLevel);
     bool isVar = false;
     bool hasModifier = false;
     if (varFinalOrConst != null) {
       hasModifier = true;
       isVar = optional('var', varFinalOrConst);
     }
@@ skipping 32 matching lines @@
     while (optional(',', token)) {
       token = parseIdentifier(token.next, context);
       token = parseFieldInitializerOpt(token);
       ++fieldCount;
     }
     Token semicolon = token;
     token = expectSemicolon(token);
     if (isTopLevel) {
       listener.endTopLevelFields(fieldCount, start, semicolon);
     } else {
-      listener.endFields(fieldCount, start, semicolon);
+      listener.endFields(fieldCount, covariantKeyword, start, semicolon);
     }
     return token;
   }
 
   Token parseTopLevelMethod(Token start, Link<Token> modifiers, Token type,
       Token getOrSet, Token name) {
     listener.beginTopLevelMethod(start, name);
     Token externalModifier;
     // TODO(johnniwinther): Move error reporting to resolution to give more
     // specific error messages.
@@ skipping 476 matching lines @@
           isField = true;
         }
         break;
       } else if ((identical(value, '=')) || (identical(value, ','))) {
         isField = true;
         break;
       } else {
         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;
         }
       }
     }
 
     var modifiers = identifiers.reverse();
     token = isField
         ? parseFields(start, modifiers, type, getOrSet, name, false)
         : parseMethod(start, modifiers, type, getOrSet, name);
@@ skipping 1811 matching lines @@
         break;
     }
     if (isRecoverable) {
       listener.handleRecoverableError(token, kind, arguments);
       return null;
     } else {
       return listener.handleUnrecoverableError(token, kind, arguments);
     }
   }
 }