Sdk: Platform.isLinux and company should be const

[Hixie]

We'd like to have code that does something like this:

// these are giant maps
const Map mac = {...};
const Map windows = {...};
const Map linux = {...};

convert(foo) {
  if (Platform.isMac)
    return mac[foo];
  if (Platform.isLinux)
    return linux[foo];
  if (Platform.isWindows)
    return windows[foo];
  return null;
}

We only want to ship the maps that apply to the current platform; we'd like the others to tree-shake out.

[Hixie]

cc @gspencergoog

[sortie]

Hi. I'm the new maintainer of dart:io. I actually ran into this issue the other day, I agree it would be nice for them to be const.

[zoechi]

Seems to be related to #31969

[gspencergoog]

Any idea on where this issue is in the priorities for the SDK? This would be pretty useful, especially now that flutter is expanding to more platforms.

[sortie]

Hi Greg. I'm hoping to do some work on this issue this quarter so flutter for web is better supported. I'm writing up a plan and will be sharing it.

[franklinyow]

@sortie Please re-evaluate this and do one of the following:

1. Add it to a milestone if it is still P1
2. Change priority label and add milestone if no longer P1
3. Close, if this issue is no longer needed