|  | library /*isNonNullableByDefault*/; | 
|  | import self as self; | 
|  | import "dart:core" as core; | 
|  |  | 
|  | class D1 extends core::Object { | 
|  | synthetic constructor •() → self::D1 | 
|  | : super core::Object::•() | 
|  | ; | 
|  | } | 
|  | class D2 extends core::Object { | 
|  | synthetic constructor •() → self::D2 | 
|  | : super core::Object::•() | 
|  | ; | 
|  | } | 
|  | class D extends core::Object implements self::D1, self::D2 { | 
|  | synthetic constructor •() → self::D | 
|  | : super core::Object::•() | 
|  | ; | 
|  | } | 
|  | class A extends core::Object { | 
|  | synthetic constructor •() → self::A | 
|  | : super core::Object::•() | 
|  | ; | 
|  | method m(covariant-by-declaration self::D d) → void {} | 
|  | } | 
|  | abstract class B1 extends core::Object { | 
|  | synthetic constructor •() → self::B1 | 
|  | : super core::Object::•() | 
|  | ; | 
|  | abstract method m(self::D1 d1) → void; | 
|  | } | 
|  | abstract class B2 extends core::Object { | 
|  | synthetic constructor •() → self::B2 | 
|  | : super core::Object::•() | 
|  | ; | 
|  | abstract method m(self::D2 d2) → void; | 
|  | } | 
|  | static method main() → dynamic {} |