Simplify pause_idle_isolate_test and add more debugging output.

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 70 matching lines @@
 }
 
 
 Future<Isolate> hasPausedFor(Isolate isolate, String kind) {
   // Set up a listener to wait for breakpoint events.
   Completer completer = new Completer();
   isolate.vm.getEventStream(VM.kDebugStream).then((stream) {
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
         if (event.kind == kind) {
-          print('Paused with $kind');
-          subscription.cancel();
           if (completer != null) {
             // Reload to update isolate.pauseEvent.
+            print('Paused with $kind');
+            subscription.cancel();
             completer.complete(isolate.reload());
             completer = null;
           }
         }
     });
 
     // Pause may have happened before we subscribed.
     isolate.reload().then((_) {
       if ((isolate.pauseEvent != null) &&
          (isolate.pauseEvent.kind == kind)) {
         // Already waiting at a breakpoint.
-        print('Paused with $kind');
-        subscription.cancel();
         if (completer != null) {
+          print('Paused with $kind');
+          subscription.cancel();
           completer.complete(isolate);
           completer = null;
         }
       }
     });
   });
 
   return completer.future;  // Will complete when breakpoint hit.
 }
 
@@ skipping 108 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;
 }