Move Platform (or similar) to dart:core

[vsmenon]

A primary reason for Flutter apps to import dart:io is to access the Platform class.

Moving this class to dart:core (or some other shared library) would simplify development for Flutter Web.

FYI - @lrhn @ferhatb @jonahwilliams @kevmoo

If we do this, we'd need to understand what these APIs should return on the Web:

https://api.dartlang.org/dev/2.5.0-dev.1.0/dart-io/Platform-class.html

E.g., does Platform.isAndroid mean native Android or would it also be true for Android Chrome, etc.

[vsmenon]

Actually, let me close this in favor of:

#35969

and retitle / label that one.