Fix a couple of bugs with async stack traces

runtime/observatory/tests/service/regress_28980_test.dart

 // Copyright (c) 2017, 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 'test_helper.dart';
 import 'service_test_common.dart';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
-const int LINE_A = 28, LINE_B = 33;
+const int LINE_A = 19, LINE_B = 38;
 
-class VMServiceClient {
-  VMServiceClient(this.x);
-  close() => new Future.microtask(() => print("close"));
-  var x;
+var _lock;
+var _lockEnabled = true;
+
+String flutterRoot = "abc";
+
+foo(a, b, c, d) {
+  return new A(); // LINE_A
 }
 
-collect() async {
-  var uri = "abc";
-  var vmService;
-  await new Future.microtask(() async {
-    try {
-      vmService = new VMServiceClient(uri);
-      await new Future.microtask(() => throw new TimeoutException("here"));
-    } on dynamic {
-      vmService.close();
-      rethrow; // LINE_A
-    }
-  });
+class A {
+  Future lock() => new Future.microtask(() => print("lock"));
+  String path = "path";
 }
 
-test_code() async {
-  // LINE_B
-  try {
-    await collect();
-  } on TimeoutException {
-    print("ok");
+class FileSystemException {}
+
+Future<Null> test_code() async {
+  if (!_lockEnabled) return null;
+  assert(_lock == null);
+  _lock = foo(flutterRoot, 'bin', 'cache', 'lockfile');
+  bool locked = false;
+  bool printed = false;
+  while (!locked) {
+    try {
+      await _lock.lock();
+      locked = true; // LINE_B
+    } on FileSystemException {
+      if (!printed) {
+        printTrace('Print path: ${_lock.path}');
+        printStatus('Just another line...');
+        printed = true;
+      }
+      await new Future<Null>.delayed(const Duration(milliseconds: 50));
+    }
   }
 }
 
 Future<Isolate> stepThroughProgram(Isolate isolate) async {
   Completer completer = new Completer();
   int pauseEventsSeen = 0;
 
   await subscribeToStream(isolate.vm, VM.kDebugStream,
       (ServiceEvent event) async {
     if (event.kind == ServiceEvent.kPauseBreakpoint) {
       // We are paused: Step further.
       pauseEventsSeen++;
       isolate.stepInto();
     } else if (event.kind == ServiceEvent.kPauseExit) {
       // We are at the exit: The test is done.
       expect(pauseEventsSeen > 20, true,
           reason: "Saw only $pauseEventsSeen pause events.");
       await cancelStreamSubscription(VM.kDebugStream);
       completer.complete();
     }
   });
   isolate.resume();
   return completer.future;
 }
 
 var tests = [
   hasPausedAtStart,
   markDartColonLibrariesDebuggable,
+  setBreakpointAtLine(LINE_A),
+  resumeIsolate,
+  hasStoppedAtBreakpoint,
   setBreakpointAtLine(LINE_B),
   resumeIsolate,
   hasStoppedAtBreakpoint,
-  setBreakpointAtLine(LINE_A),
+  stepInto,
+  stepInto,
+  stepInto,
   resumeIsolate,
-  hasStoppedAtBreakpoint,
-  stepOut,
-  stoppedAtLine(LINE_B),
-  resumeIsolate
 ];
 
 main(args) => runIsolateTestsSynchronous(args, tests,
     testeeConcurrent: test_code, pause_on_start: true, pause_on_exit: false);