Elements. Set element for a synthetic covariant merge.

Change-Id: I61e7ba6c6753811eedf75acdfdfcc294467dc8e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404200
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/element/inheritance_manager3.dart

@@ -1071,6 +1071,11 @@ class InheritanceManager3 {
       result.parameters = transformedParameters;
       result.returnType = executable.returnType;
       result.typeParameters = executable.typeParameters.cast();
+      result.element = MethodElementImpl2(
+        Reference.root(), // TODO(scheglov): wrong
+        executable.name,
+        result,
+      );
       return result;
     }