Support for the 'covariant' keyword for formal parameters.

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 65f41aed1056906e6b6fc50ace4721f84942a2de..2a58ae6dbfcc3a8c525a84b269a5f7552ef7dc96 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -499,8 +499,10 @@ class Parser {
     // modifier.
     // This enables the case where `covariant` is the name of the parameter:
     //    void foo(covariant);
+    Token covariantKeyword;
     if (identical(token.stringValue, 'covariant') &&
         (token.next.isIdentifier() || isModifier(token.next))) {
+      covariantKeyword = token;
       token = token.next;
     }
     token = parseModifiers(token);
@@ -574,7 +576,7 @@ class Parser {
     } else {
       listener.handleFormalParameterWithoutValue(token);
     }
-    listener.endFormalParameter(thisKeyword, kind);
+    listener.endFormalParameter(covariantKeyword, thisKeyword, kind);
     return token;
   }