Support for the 'covariant' keyword for formal parameters.

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

 // Copyright (c) 2016, 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.body_builder;
 
 import 'package:front_end/src/fasta/parser/parser.dart' show
     FormalParameterType,
     optional;
 
@@ skipping 1259 matching lines @@
   }
 
   @override
   void endThrowExpression(Token throwToken, Token endToken) {
     debugEvent("ThrowExpression");
     Expression expression = popForValue();
     push(new Throw(expression));
   }
 
   @override
-  void endFormalParameter(Token thisKeyword, FormalParameterType kind) {
+  void endFormalParameter(Token covariantKeyword, Token thisKeyword,
+      FormalParameterType kind) {
     debugEvent("FormalParameter");
     // TODO(ahe): Need beginToken here.
     int charOffset = thisKeyword?.charOffset;
     if (thisKeyword != null) {
       if (!inConstructor) {
         addCompileTimeError(thisKeyword.charOffset,
             "'this' parameters can only be used on constructors.");
         thisKeyword = null;
       }
     }
@@ skipping 1412 matching lines @@
   } else if (node is TypeDeclarationBuilder) {
     return node.name;
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisPropertyAccessor) {
     return node.name.name;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }