The ui.PlatformDispatcher to which this binding is bound.
A number of additional bindings are defined as extensions of BindingBase, e.g., ServicesBinding, RendererBinding, and WidgetsBinding. Each of these bindings define behaviors that interact with a ui.PlatformDispatcher, e.g., ServicesBinding registers listeners with the ChannelBuffers, and RendererBinding registers ui.PlatformDispatcher.onMetricsChanged, ui.PlatformDispatcher.onTextScaleFactorChanged, ui.PlatformDispatcher.onSemanticsEnabledChanged, and ui.PlatformDispatcher.onSemanticsAction handlers.
Each of these other bindings could individually access a ui.PlatformDispatcher statically, but that would preclude the ability to test these behaviors with a fake platform dispatcher for verification purposes. Therefore, BindingBase exposes this ui.PlatformDispatcher for use by other bindings. A subclass of BindingBase, such as TestWidgetsFlutterBinding, can override this accessor to return a different ui.PlatformDispatcher implementation.
@override final TestPlatformDispatcher platformDispatcher;