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

lib/src/utils.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 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.
 typedef AsyncFunction();
 
 /// A regular expression to match the exception prefix that some exceptions'
 /// [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
 /// [toString], so we remove that if it exists.
 String getErrorMessage(error) =>
   error.toString().replaceFirst(_exceptionPrefix, '');
 
 /// Indent each line in [str] by two spaces.
 String indent(String str) =>
     str.replaceAll(new RegExp("^", multiLine: true), "  ");
@@ skipping 77 matching lines @@
 
   // Otherwise truncate to return the trailing text, but attempt to start at
   // the beginning of a word.
   var result = text.substring(text.length - maxLength + 4);
   var firstSpace = result.indexOf(' ');
   if (firstSpace > 0) {
     result = result.substring(firstSpace);
   }
   return '...$result';
 }