Support for the 'covariant' keyword for formal parameters.

pkg/front_end/lib/src/fasta/source/outline_builder.dart

Index: pkg/front_end/lib/src/fasta/source/outline_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/outline_builder.dart b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
index 557a9e56968e34b62683f69f71706ee37b570bfe..5d56599c8a8d4c7901f65953308c3c38cd6bc115 100644
--- a/pkg/front_end/lib/src/fasta/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
@@ -372,7 +372,8 @@ class OutlineBuilder extends UnhandledListener {
   }
 
   @override
-  void endFormalParameter(Token thisKeyword, FormalParameterType kind) {
+  void endFormalParameter(Token covariantKeyword, Token thisKeyword,
+      FormalParameterType kind) {
     debugEvent("FormalParameter");
     String name = pop();
     TypeBuilder type = pop();