Add a compile-time constant for platform

Currently there are few ways to check for the platform. They are mostly runtime and have downsides. Ways to check for different platforms are inconsistent.

- `kIsWeb` of `flutter/foundation` is the most accepted way to check if the code is built for web. It translates to `const bool kIsWeb = identical(0, 0.0);` which is not future-proof. Who knows what other platforms Dart may support in the future that do not tell int from double?
- For other platforms we have fields like `Platform.isAndroid` of `dart:io` which is not supported in web. These translate to `static final bool isAndroid = (_operatingSystem == "android");` They take runtime comparison of strings which is not a first idea of efficiency when something is supposed to never change.

A cleaner and faster solution would be to have one constant for a platform so it could be used in a `switch` so that a compiler could drop unneeded branches for every platform.

Dart uses different compilers for different platforms, doesn't it? So it should be possible?

Start by comparing the existing kIsWeb and dart:io Platform.isAndroid checks described in the issue, then review how Dart's different compilers could expose a compile-time platform value. No file or test is named; the work is complete only when the language-design and compiler implications are specified and accepted.

Written by the indexing model from the issue text.