Don't use a special var descriptor for :async_op since it can now be either stack or context alloca…

runtime/observatory/tests/service/step_into_async_no_await_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.
 // VMOptions=--error_on_bad_type --error_on_bad_override  --verbose_debug
 
 import 'package:observatory/service_io.dart';
 import 'test_helper.dart';
 import 'dart:developer';
 
-foo() async { }
-
-doAsync(stop) async {
-  if (stop) debugger();
-  await foo(); // Line 14.
-  await foo(); // Line 15.
-  await foo(); // Line 16.
-  return null;
+// :async_op will not be captured in this function because it never needs to
+// reschedule it.
+asyncWithoutAwait() async {
+  print("asyncWithoutAwait");
 }
 
 testMain() {
-  // With two runs of doAsync floating around, async step should only cause
-  // us to stop in the run we started in.
-  doAsync(false);
-  doAsync(true);
-}
-
-asyncNext(Isolate isolate) async {
-  return isolate.asyncStepOver()[Isolate.kSecondResume];
+  debugger();
+  asyncWithoutAwait();
 }
 
 var tests = [
   hasStoppedAtBreakpoint,
-  stoppedAtLine(14),
-  asyncNext,
+  stoppedAtLine(18),
+  (isolate) => isolate.stepInto(),
   hasStoppedAtBreakpoint,
-  stoppedAtLine(15),
-  asyncNext,
-  hasStoppedAtBreakpoint,
-  stoppedAtLine(16),
+  (isolate) => isolate.getStack(),  // Should not crash.
+  // TODO(rmacnak): stoppedAtLine(12)
+  // This doesn't happen because asyncWithoutAwait is marked undebuggable.
+  // Probably needs to change to support async-step-into.
   resumeIsolate,
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);