Simplify experimental pedometer example

[liamappelbe]

Since ffigen added support for NativeCallable.listener to its ObjC bindings, this example can be simplified. We can replace the Dart_Port logic with ObjCBlock.listener, which lets us get rid of most of the native code.

We still need a small bit of native code to retain a reference to the callback's arguments before invoking the listener, otherwise the arguments may be ref counted and deleted before the Dart side of the callback is invoked. See dart-lang/native#835

[liamappelbe]

Nevermind, I think this is blocked on another small tweak to the ObjC bindings.

[liamappelbe]

Actually I managed to fix the bug without ffigen changes. Ready for review.

[domesticmouse]

Looks like there are CI issues:

warning - example/lib/steps_repo.dart:5:8 - Unused import: 'dart:isolate'. Try removing the import directive. - unused_import
warning - example/lib/steps_repo.dart:102:49 - The receiver can't be null, so the null-aware operator '?.' is unnecessary. Try replacing the operator '?.' with '.'. - invalid_null_aware_operator
warning - example/lib/steps_repo.dart:104:61 - The '!' will have no effect because the receiver can't be null. Try removing the '!' operator. - unnecessary_non_null_assertion

[liamappelbe]

Looks like there are CI issues:

Fixed

[domesticmouse]

PTAL @leighajarett