deferredComponent constant Null safety

A MethodChannel for installing and managing deferred components.

The following outgoing methods are defined for this channel (invoked using OptionalMethodChannel.invokeMethod):

• installDeferredComponent: Requests that a deferred component identified by the provided loadingUnitId or componentName be downloaded and installed. Providing a loadingUnitId with null componentName will install a component that includes the desired loading unit. If a componentName is provided, then the deferred component with the componentName will be installed. This method returns a future that will not be completed until the feature is fully installed and ready to use. When an error occurs, the future will complete an error. Calling loadLibrary() on a deferred imported library is equivalent to calling this method with a loadingUnitId and null componentName.
• uninstallDeferredComponent: Requests that a deferred component identified by the provided loadingUnitId or componentName be uninstalled. Since uninstallation typically requires significant disk i/o, this method only signals the intent to uninstall. Actual uninstallation (eg, removal of assets and files) may occur at a later time. However, once uninstallation is requested, the deferred component should not be used anymore until installDeferredComponent or loadLibrary is called again.