Make catch_errors13_test non-flaky.

tests/lib/async/catch_errors13_test.dart

 // Copyright (c) 2013, 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 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'catch_errors.dart';
 
 main() {
   asyncStart();
   Completer done = new Completer();
 
   var events = [];
   // Work around bug that makes scheduleMicrotask use Timers. By invoking
   // `scheduleMicrotask` here we make sure that asynchronous non-timer events
   // are executed before any Timer events.
   scheduleMicrotask(() { });
 
   // Test that errors are caught by nested `catchErrors`. Also uses
   // `scheduleMicrotask` in the body of a Timer.
+  bool outerTimerRan = false;
+  int outerTimerCounterDelayCount = 0;
   catchErrors(() {
     events.add("catch error entry");
     catchErrors(() {
       events.add("catch error entry2");
       Timer.run(() { throw "timer error"; });
-      new Timer(const Duration(milliseconds: 50),
-                () {
-                     scheduleMicrotask(() { throw "scheduleMicrotask"; });
-                     throw "delayed error";
-                   });
+
+      // We want this timer to run after the timer below. When the machine is
+      // slow we saw that the timer below was scheduled more than 50ms after
+      // this line. A duration of 50ms was therefore not enough to guarantee
+      // that this timer runs before the timer below.
+      // Instead of increasing the duration to a bigger amount we decided to
+      // verify that the other timer already ran, and reschedule if not.
+      // This way we could reduce the timeout (to 10ms now), while still
+      // allowing for more time, in case the machine is slow.
+      void runDelayed() {
+        new Timer(const Duration(milliseconds: 10),
+                  () {
+                    if (outerTimerRan) {
+                      scheduleMicrotask(() { throw "scheduleMicrotask"; });
+                      throw "delayed error";
+                    } else if (outerTimerCounterDelayCount < 100) {
+                      outerTimerCounterDelayCount++;
+                      runDelayed();
+                    }
+                  });
+      }
+
+      runDelayed();
     }).listen((x) {
       events.add(x);
       if (x == "scheduleMicrotask") {
         throw "inner done throw";
       }
     });
     events.add("after inner");
-    Timer.run(() { throw "timer outer"; });
+    Timer.run(() {
+      outerTimerRan = true;
+      throw "timer outer";
+    });
     throw "inner throw";
   }).listen((x) {
       events.add(x);
       if (x == "inner done throw") done.complete(true);
     },
     onDone: () { Expect.fail("Unexpected callback"); });
 
   done.future.whenComplete(() {
     // Give callbacks time to run.
     Timer.run(() {
       Expect.listEquals([
                          "catch error entry",
                          "catch error entry2",
                          "after inner",
                          "main exit",
                          "inner throw",
                          "timer error",
                          "timer outer",
                          "delayed error",
                          "scheduleMicrotask",
                          "inner done throw"
                          ],
                          events);
       asyncEnd();
     });
   });
   events.add("main exit");
 }