Added a test for issue 22523.

tests/language/async_rethrow_test.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.
+
+import "dart:async";
+import "package:expect/expect.dart";
+
+const String exceptionString = "exceptionalString";
+
+throwString() async {
+  try {
+    throw exceptionString;
+  } catch (e) {
+    await 1;
+    throw e;
+  }
+}
+
+rethrowString() async {
+  try {
+    throw exceptionString;
+  } catch (e) {
+    await 1;
+    rethrow;
+  }
+}
+
+testThrow() {
+  Future f = throwString();
+  f.then((v) { Expect.fail("Exception not thrown"); },
+      onError: (e) { Expect.equals(exceptionString, e);});
+}
+
+
+testRethrow() {
+  Future f = rethrowString();
+  f.then((v) { Expect.fail("Exception not thrown"); },
+      onError: (e) { Expect.equals(exceptionString, e);});
+}
+
+main() {
+  testThrow();
+  testRethrow();
+}