Support for the 'covariant' keyword for formal parameters.

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

Index: pkg/compiler/lib/src/parser/node_listener.dart
diff --git a/pkg/compiler/lib/src/parser/node_listener.dart b/pkg/compiler/lib/src/parser/node_listener.dart
index 0cd290a50fdabc3995957f48179545319739f8c3..6a71b8375b2262f272f435870f9c4730ae1e3c6a 100644
--- a/pkg/compiler/lib/src/parser/node_listener.dart
+++ b/pkg/compiler/lib/src/parser/node_listener.dart
@@ -231,7 +231,8 @@ class NodeListener extends ElementListener {
   }
 
   @override
-  void endFormalParameter(Token thisKeyword, FormalParameterType kind) {
+  void endFormalParameter(Token covariantKeyword, Token thisKeyword,
+      FormalParameterType kind) {
     Expression name = popNode();
     if (thisKeyword != null) {
       Identifier thisIdentifier = new Identifier(thisKeyword);