Small fix in tests/light_unittest.dart

dart/tests/light_unittest.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.
 
 /// A light-weight replacement for package:unittest.  This library runs tests
 /// synchronously, and avoids using reflection.
 library light_unittest;
 
 import 'dart:async';
 
@@ skipping 41 matching lines @@
   }
 }
 
 get throws => new Expectation((actual) => Expect.throws(actual));
 
 get isTrue => new Expectation((actual) => Expect.isTrue(actual));
 
 expectAsync1(then) {
   asyncStart();
   return (x) {
+    // 'then(x)' may call 'asyncStart()', so we first need to execute it, before
+    // we can call 'asyncEnd()'.
+    var result = then(x);
     asyncEnd();
-    return then(x);
+    return result;
   };
 }