Small fix in tests/light_unittest.dart

dart/tests/light_unittest.dart

Index: dart/tests/light_unittest.dart
diff --git a/dart/tests/light_unittest.dart b/dart/tests/light_unittest.dart
index 02ed4332240f94b2b46f48a0c7d1ad7e810bdc99..d47919f6ea541a212d7c8f0ab75701fb03538b17 100644
--- a/dart/tests/light_unittest.dart
+++ b/dart/tests/light_unittest.dart
@@ -59,7 +59,10 @@ 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;
   };
 }