Make APIs in `package:platform` that wrap `dart:io` / `dart:html` and are amenable to tree shaking by compilers

[mkustermann]

Once package:platform has the necessary APIs, we should update Flutter documentation of defaultTargetPlatform to recommend using package:platform if one wants tree shaking.

[mit-mit]

https://github.com/dart-lang/platform and https://pub.dev/packages/platform have been transferred to Dart in preparation for the upcoming API updates.

[sstrickl]

So currently using defaultTargetPlatform as a guard for platform-dependent code ensures appropriate tree-shaking, but package:platform does not. How the former was handled in flutter/flutter#141105 may be useful when updating this package to use @pragma('vm:platform-const-if', ...) appropriately to allow for testing while still tree-shaking in release code.