Add a testOn parameter to test() and group().

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 6b2b21ef6c22f648490648009ec79d4ab2f70731..2a12238932ded4d1533bd5c9b34f3094e3012713 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -6,8 +6,11 @@ library unittest.utils;
 
 import 'dart:async';
 
+import 'package:path/path.dart' as p;
 import 'package:stack_trace/stack_trace.dart';
 
+import 'backend/operating_system.dart';
+
 /// A typedef for a possibly-asynchronous function.
 ///
 /// The return type should only ever by [Future] or void.
@@ -17,6 +20,30 @@ typedef AsyncFunction();
 /// [Object.toString] values contain.
 final _exceptionPrefix = new RegExp(r'^([A-Z][a-zA-Z]*)?(Exception|Error): ');
 
+/// Directories that are specific to OS X.
+///
+/// This is used to try to distinguish OS X and Linux in [currentOsGuess].
+final _macOsDirectories = new Set<String>.from([
+  "/Applications",
+  "/Library",
+  "/Network",
+  "/System",
+  "/Users"
+]);
+
+/// Returns the best guess for the current operating system without using
+/// `dart:io`.
+///
+/// This is useful for running test files directly and skipping tests as
+/// appropriate. The only OS-specific information we have is the current path,
+/// which we try to use to figure out the OS.
+final OperatingSystem currentOsGuess = (() {
+  if (p.style == p.Style.url) return OperatingSystem.none;
+  if (p.style == p.Style.windows) return OperatingSystem.windows;
+  if (_macOsDirectories.any(p.current.startsWith)) return OperatingSystem.macOs;
+  return OperatingSystem.linux;
+})();
+
 /// Get a string description of an exception.
 ///
 /// Many exceptions include the exception class name at the beginning of their