Add heart beat tests to make sure we see an isolate's sticky error after exit

runtime/observatory/tests/service/service_test_common.dart

 // Copyright (c) 2016, 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.
 
 library service_test_common;
 
 import 'dart:async';
 import 'package:observatory/service_common.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 122 matching lines @@
 }
 
 Future<Isolate> hasStoppedAtBreakpoint(Isolate isolate) {
   return hasPausedFor(isolate, ServiceEvent.kPauseBreakpoint);
 }
 
 Future<Isolate> hasStoppedWithUnhandledException(Isolate isolate) {
   return hasPausedFor(isolate, ServiceEvent.kPauseException);
 }
 
+Future<Isolate> hasStoppedAtExit(Isolate isolate) {
+  return hasPausedFor(isolate, ServiceEvent.kPauseExit);
+}
+
 Future<Isolate> hasPausedAtStart(Isolate isolate) {
   return hasPausedFor(isolate, ServiceEvent.kPauseStart);
 }
 
 // Currying is your friend.
 IsolateTest setBreakpointAtLine(int line) {
   return (Isolate isolate) async {
     print("Setting breakpoint for line $line");
     Library lib = await isolate.rootLibrary.load();
     Script script = lib.scripts.single;
@@ skipping 90 matching lines @@
 
 Future<Instance> rootLibraryFieldValue(Isolate isolate,
                                        String fieldName) async {
   Library rootLib = await isolate.rootLibrary.load();
   Field field = rootLib.variables.singleWhere((v) => v.name == fieldName);
   await field.load();
   Instance value = field.staticValue;
   await value.load();
   return value;
 }