Fix async machinery (issue 22445 and possibly others to be triaged later).

tests/language/async_finally_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";
+
+foo() async {
+  try {
+    await 1;
+    throw "error";
+  } on String catch (e) {
+    await 2;
+    throw e;
+  } finally {
+    await 3;
+  }
+}
+
+main() async {
+  var error = "no error";
+  try {
+    await foo();
+  } catch (e) {
+    error = e;
+  }
+  Expect.equals("error", error);
+}
+