Add a Loader class for loading tests from files.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
index f9b4075d70df066393024421491889b9a0561c3b..88845833403e09d084198bfe3d0158bb2ca9e606 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -6,6 +6,10 @@ library unittest.test.utils;
 
 import 'dart:async';
 import 'dart:collection';
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+import 'package:stack_trace/stack_trace.dart';
 
 import 'package:unittest/src/live_test.dart';
 import 'package:unittest/src/remote_exception.dart';
@@ -15,6 +19,13 @@ import 'package:unittest/unittest.dart';
 // The last state change detected via [expectStates].
 State lastState;
 
+final String packageDir = _computePackageDir();
+
+String _computePackageDir() {
+  var trace = new Trace.current();
+  return p.dirname(p.dirname(p.fromUri(trace.frames.first.uri)));
+}
+
 /// Asserts that exactly [states] will be emitted via [liveTest.onStateChange].
 ///
 /// The most recent emitted state is stored in [_lastState].
@@ -71,6 +82,12 @@ Matcher isRemoteException(String message) => predicate(
     (error) => error is RemoteException && error.message == message,
     'is a RemoteException with message "$message"');
 
+/// Returns a matcher that matches a [FileSystemException] with the given
+/// [message].
+Matcher isFileSystemException(String message) => predicate(
+    (error) => error is FileSystemException && error.message == message,
+    'is a FileSystemException with message "$message"');
+
 /// Returns a [Future] that completes after pumping the event queue [times]
 /// times.
 ///