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

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 362a70720d3982d4657067236374145ae9ad46ef..2fbee68da60f9a319ffb4101d534fa337b768e3a 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -4594,6 +4594,9 @@ class FieldElementImpl extends PropertyInducingElementImpl
    * Return `true` if this field was explicitly marked as being covariant.
    */
   bool get isCovariant {
+    if (_kernel != null) {
+      return _kernel.isCovariant;
+    }
     if (_unlinkedVariable != null) {
       return _unlinkedVariable.isCovariant;
     }
@@ -8218,6 +8221,9 @@ class ParameterElementImpl extends VariableElementImpl
    * Return true if this parameter is explicitly marked as being covariant.
    */
   bool get isExplicitlyCovariant {
+    if (_kernel != null) {
+      return _kernel.isCovariant;
+    }
     if (_unlinkedParam != null) {
       return _unlinkedParam.isExplicitlyCovariant;
     }