Add on_os and on_platform fields.

lib/src/backend/operating_system.dart

Index: lib/src/backend/operating_system.dart
diff --git a/lib/src/backend/operating_system.dart b/lib/src/backend/operating_system.dart
index 45b14133f22f92d5390f70070a298ecba5c88aee..0e9c63784a1fa5abe231b41502a55edaca76c6ab 100644
--- a/lib/src/backend/operating_system.dart
+++ b/lib/src/backend/operating_system.dart
@@ -42,7 +42,8 @@ class OperatingSystem {
   ///
   /// If no operating system is found, returns [none].
   static OperatingSystem find(String name) =>
-      all.firstWhere((platform) => platform.name == name, orElse: () => null);
+      all.firstWhere((platform) => platform.identifier == name,
+      orElse: () => null);
 
   /// Finds an operating system by the return value from `dart:io`'s
   /// `Platform.operatingSystem`.