Store the covariant keyword flag for parameters and fields into Kernel and resynthesize in analyzer.

pkg/front_end/lib/src/fasta/builder/modifier_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart b/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
index 114a9f680a6d125ccf3ee658ac859ec203fd16bf..8af347b0ba153c2fa71804d9a80dbaa1ba08c75c 100644
--- a/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
@@ -8,6 +8,7 @@ import '../modifier.dart'
     show
         abstractMask,
         constMask,
+        covariantMask,
         externalMask,
         finalMask,
         namedMixinApplicationMask,
@@ -27,6 +28,8 @@ abstract class ModifierBuilder extends Builder {
 
   bool get isConst => (modifiers & constMask) != 0;
 
+  bool get isCovariant => (modifiers & covariantMask) != 0;
+
   bool get isExternal => (modifiers & externalMask) != 0;
 
   bool get isFinal => (modifiers & finalMask) != 0;