anchor.diff:  /// in the language tour.
   library meta;
   
  +/// Used to annotate a parameter of an instance method that overrides another
  +/// method.
  +///
  +/// Indicates that this parameter may have a tighter type than the parameter on
  +/// its superclass. The actual argument will be checked at runtime to ensure it
  +/// is a subtype of the overridden parameter type.
  +const _Checked checked = const _Checked();
  +
   /// Used to annotate an instance or static method `m`. Indicates that `m` must
   /// either be abstract or must return a newly allocated object or `null`. In
   /// addition, every method that either implements or overrides `m` is implicitly
  
positive.code: /// * the declaration is referenced outside of its the defining library or a
  ///   library which is in the `test` folder of the defining package.
  const _VisibleForTesting visibleForTesting = const _VisibleForTesting();
  
  /// Used to annotate a named parameter `p` in a method or function `f`.
  ///
  
negative.code: ///   corresponding to a named parameter that has this annotation.
  const Required required = const Required();
  
  /// Used to annotate a field is allowed to be overridden in Strong Mode.
  const _Virtual virtual = const _Virtual();
  
  /// Used to annotate a declaration was made public, so that it is more visible
  /// than otherwise necessary, to make code testable.
  ///
  