The parameter of synthetic setters should inherit 'covariant'.

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 aa8ce252ba64123d7c7c42f46fd934bdb6394b0d..b3c54c02211149048badf8766c014426b6551d53 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -7800,6 +7800,16 @@ class ParameterElementImpl_ofImplicitSetter extends ParameterElementImpl {
   }
 
   @override
+  bool get inheritsCovariant {
+    PropertyInducingElement variable = setter.variable;
+    if (variable is FieldElementImpl && variable._unlinkedVariable != null) {
+      return enclosingUnit.resynthesizerContext
+          .inheritsCovariant(variable._unlinkedVariable.inheritsCovariantSlot);
+    }
+    return super.inheritsCovariant;
+  }
+
+  @override
   bool get isCovariant {
     if (isExplicitlyCovariant || inheritsCovariant) {
       return true;