anchor.diff:    /// See also:
     ///
     ///  * <https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#rpcs-requests-and-responses>
  +  @protected
  +  @mustCallSuper
     void initServiceExtensions() {
       assert(!_debugServiceExtensionsRegistered);
       registerSignalServiceExtension(
  
positive.code:   /// be exposed as `MixinClassName.instance`, a static getter that returns
    /// `MixinClassName._instance`, a static field that is set by
    /// `initInstances()`.
    void initInstances() {
      assert(!_debugInitialized);
      assert(() { _debugInitialized = true; return true; });
  
negative.code:   ///
    /// Calls the `setter` callback with the new value when the
    /// service extension method is called with a new value.
    @protected
    void registerBoolServiceExtension({
      String name,
      @required ValueGetter<bool> getter,
  