Fix restoring of async try context in catch clause (issue 23498).

tests/language/regress_23498_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;
-  } catch(e) {
+    try {
+      await new Future.error('error');
+    } catch (error) {
+      print("caught once");
+      throw 'error';
+    }
+  } catch (error) {
+    print("caught twice");
+    throw 'error';
   }
-  throw "error";
 }
 
 main() async {
   var error = "no error";
   try {
     await foo();
   } catch (e) {
     error = e;
   }
   Expect.equals("error", error);
 }