Add a Loader class for loading tests from files.

lib/src/io.dart

 // Copyright (c) 2015, 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.io;
 
+import 'dart:async';
 import 'dart:io';
 
 /// Whether "special" strings such as Unicode characters or color escapes are
 /// safe to use.
 ///
 /// On Windows or when not printing to a terminal, only printable ASCII
 /// characters should be used.
 bool get canUseSpecialChars =>
     Platform.operatingSystem != 'windows' &&
     stdioType(stdout) == StdioType.TERMINAL;
 
 /// Gets a "special" string (ANSI escape or Unicode).
 ///
 /// On Windows or when not printing to a terminal, returns something else since
 /// those aren't supported.
 String getSpecial(String special, [String onWindows = '']) =>
     canUseSpecialChars ? special : onWindows;
+
+/// Creates a temporary directory and passes its path to [fn].
+///
+/// Once the [Future] returned by [fn] completes, the temporary directory and
+/// all its contents are deleted. [fn] can also return `null`, in which case
+/// the temporary directory is deleted immediately afterwards.
+///
+/// Returns a future that completes to the value that the future returned from
+/// [fn] completes to.
+Future withTempDir(Future fn(String path)) {
+  return new Future.sync(() {
+    // TODO(nweiz): Empirically test whether sync or async functions perform
+    // better here when starting a bunch of isolates.
+    var tempDir = Directory.systemTemp.createTempSync('unittest_');
+    return new Future.sync(() => fn(tempDir.path))
+        .whenComplete(() => tempDir.deleteSync(recursive: true));
+  });
+}