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

pkg/front_end/testcases/runtime_checks/dynamic_invocation.dart.direct.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C<T extends core::Object> extends core::Object {
   default constructor •() → void
     : super core::Object::•()
     ;
   method f1(self::C::T x) → void {}
   method f2(core::int x) → void {}
 }
 class D extends self::C<core::num> {
   default constructor •() → void
     : super self::C::•()
     ;
-  method f1(core::int x) → void {}
+  method f1(covariant core::int x) → void {}
 }
 static method g1(dynamic d) → void {
   d.f1(1.5);
 }
 static method g2(dynamic d) → void {
   d.f2(1.5);
 }
 static method test() → void {
   self::g1(new self::C::•<core::int>());
   self::g2(new self::C::•<dynamic>());
   self::g1(new self::D::•());
 }
 static method main() → dynamic {}