Add OS constants for use in switch statements

[gspencergoog]

This adds constants for each of the platforms so that you can write:

switch (platform.operatingSystem) {
  case Platform.macOS:
	// ...
	break;
  case Platform.linux:
	// ...
	break;
  // ...
}

Instead of having to know what the exact case-sensitive strings are that are possible values for operatingSystem, you can now use these constants in conditionals, and you can access the list of possible values from Platform.operatingSystemValues.

Sure, they're still strings, but you can treat them a little more like enum values (without the switch statement validation, sadly).

[gspencergoog]

cc @tvolkert

[tvolkert]

Man, I wish we could re-write the dart:io API surface from scratch...

[gspencergoog]

@tvolkert this is ready to submit now (I fixed the formatting)