anchor.diff:  /// Specialization for check on 'num?'.
   /// Called from generated code.
   num /*?*/ _checkNumNullable(object) {
  -  if (object is num) return object;
  +  if (_isNum(object)) return _Utils.asNum(object);
     if (object == null) return object;
     throw _TypeError.forType(object, 'num');
   }
  
positive.code: /// Specialization for check on 'double?'.
  /// Called from generated code.
  double /*?*/ _checkDoubleNullable(object) {
    if (object is double) return object;
    if (object == null) return object;
    throw _TypeError.forType(object, 'double');
  }
  
negative.code: /// Specialization for check on 'String?'.
  /// Called from generated code.
  String /*?*/ _checkStringNullable(object) {
    if (_isString(object)) return _Utils.asString(object);
    if (object == null) return object;
    throw _TypeError.forType(object, 'String');
  }
  