The selector type is private

[stuartmorgan-g]

As a follow-up to the discussion in #1580 and the changes there: getting a selector is now public, but the type it returns is not; as a client of objective_c, the type returned by toSelector() is Pointer<_ObjCSelector>. This has some very problematic properties, including:

• I cannot declare a variable that stores a selector that is fully typed, which in turn means I cannot declare such a variable in flutter/packages (or flutter/flutter) without an // ignore directive.
• I cannot mock any generated objects in unit tests without a bunch of extra work due to errors like:

   Invalid @GenerateMocks annotation: Mockito cannot generate a valid mock class which implements 'NSBundle' for the following reasons:
       The method 'NSObject.doesNotRecognizeSelector_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).
       The method 'NSObject.forwardingTargetForSelector_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).
       The method 'NSObject.methodForSelector_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).
       The method 'NSObject.methodSignatureForSelector_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).
       The method 'NSObject.performSelector_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).
       The method 'NSObject.performSelector_withObject_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).
       The method 'NSObject.performSelector_withObject_withObject_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).
       The method 'NSObject.respondsToSelector_' features a private type argument, and cannot be stubbed. Try generating this mock with a MockSpec with 'unsupportedMembers' or a dummy generator (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).