anchor.diff:        //   the nullability is meaningless (ie. class A extends B) where B is the
         //   InterfaceType.
         // * The InterfaceType is the Null type.
  +      // * Non-null constructor calls.
         return typeRep;
       }
   
  
positive.code:       var gen = emitGeneratorFn((_) => [_asyncStarController]);
  
        var returnType = _expectedReturnType(function, _coreTypes.streamClass);
        var asyncStarImpl =
            InterfaceType(_asyncStarImplClass, Nullability.legacy, [returnType]);
        return js.call('new #.new(#).stream', [_emitType(asyncStarImpl), gen]);
      }
  
      assert(function.asyncMarker == AsyncMarker.Async);
  
negative.code: 
    js_ast.Expression _emitMapImplType(InterfaceType type, {bool identity}) {
      var typeArgs = type.typeArguments;
      if (typeArgs.isEmpty) {
        return _emitInterfaceType(type, emitNullability: false);
      }
      identity ??= _typeRep.isPrimitive(typeArgs[0]);
      var c = identity ? _identityHashMapImplClass : _linkedHashMapImplClass;
      return _emitInterfaceType(InterfaceType(c, Nullability.legacy, typeArgs),
          emitNullability: false);
    }
  
    js_ast.Expression _emitSetImplType(InterfaceType type, {bool identity}) {
      var typeArgs = type.typeArguments;
      if (typeArgs.isEmpty) {
        return _emitInterfaceType(type, emitNullability: false);
      }
      identity ??= _typeRep.isPrimitive(typeArgs[0]);
      var c = identity ? _identityHashSetImplClass : _linkedHashSetImplClass;
      return _emitInterfaceType(InterfaceType(c, Nullability.legacy, typeArgs),
          emitNullability: false);
    }
  
    js_ast.Expression _emitObjectLiteral(Arguments node) {
  