Ensure that descriptors have a prototype.

tests/compiler/dart2js_extra/22776.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";
+// Regression test for http://dartbug.com/22776/
+
 import "package:expect/expect.dart";
+class A {}
 
-foo() async {
+main() {
   try {
-    await 1;
-  } catch(e) {
+    print(id(new Duration(milliseconds: 10)));
+    print(id(3) ~/ new A());
+  } catch (e) {
+    print("Error '$e' ${e.runtimeType}");
   }
-  throw "error";
 }
 
-main() async {
-  var error = "no error";
-  try {
-    await foo();
-  } catch (e) {
-    error = e;
-  }
-  Expect.equals("error", error);
-}
+@AssumeDynamic()
+@NoInline()
+id(x) => x;