Add prints to help diagnose flaky async_step_test failure.

runtime/observatory/tests/service/async_step_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=--compile_all --error_on_bad_type --error_on_bad_override  --verbose_debug
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'dart:developer';
 
@@ skipping 10 matching lines @@
 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);
 }
 
 
 asyncStep(Isolate isolate) async {
   var event = isolate.pauseEvent;
+  print("Pause event is $event");
   expect(event, isNotNull);
 
   // 1. Set breakpoint for the continuation and resume the isolate.
   Instance continuation = event.asyncContinuation;
   print("Async continuation is $continuation");
+  if (continuation == null) {
+    print(await isolate.getStack());
+  }
   expect(continuation.isClosure, isTrue);
 
   var bpt = await isolate.addBreakOnActivation(continuation);
+  print("Async step to $bpt");
   expect(bpt is Breakpoint, isTrue);
-  print("Async step to $bpt");
 
   await isolate.resume();
   await hasStoppedAtBreakpoint(isolate);
+  print("Big step to: ${isolate.pauseEvent}");
 
   // 2. Step past the state-machine dispatch.
   await isolate.stepOver();
   await hasStoppedAtBreakpoint(isolate);
+  print("Small step to: ${isolate.pauseEvent}");
 }
 
 var tests = [
   hasStoppedAtBreakpoint,
   stoppedAtLine(15),
   asyncStep,
   stoppedAtLine(16),
   asyncStep,
   stoppedAtLine(17),
   resumeIsolate,
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);