Make generic type of timeout-future the same as the one from the original.

tests/lib/async/future_timeout_test.dart

Index: tests/lib/async/future_timeout_test.dart
diff --git a/tests/lib/async/future_timeout_test.dart b/tests/lib/async/future_timeout_test.dart
index 8209b8b982c2ab2a4a489cecf33bd25be1dd817f..806b4b4f427b3f19871e93787acf0f122fc6dba5 100644
--- a/tests/lib/async/future_timeout_test.dart
+++ b/tests/lib/async/future_timeout_test.dart
@@ -185,4 +185,14 @@ main() {
       expect(s, null);
     }));
   });
+
+  test("timeoutType", () {
+    Completer completer = new Completer<int>();
+    Future timedOut = completer.future.timeout(
+        const Duration(milliseconds: 5));
+    expect(timedOut, new isInstanceOf<Future<int>>());
+    expect(timedOut, isNot(new isInstanceOf<Future<String>>()));
+    timedOut.catchError((_) {});
+    completer.complete(499);
+  });
 }