Fixes #2361. Fix flaky tests on web-platforms

LibTest/async/Future/Future.delayed_A01_t02.dart

@@ -7,21 +7,30 @@
 ///    The computation will be executed after the given duration has passed, and
 /// the future is completed with the result. If the duration is 0 or less, it
 /// completes no sooner than in the next event-loop iteration.
+///
 /// @description Checks that execution of the supplied computation() function
 /// happens after delay.
 /// @author kaigorodov
 
 import "dart:async";
 import "../../../Utils/expect.dart";
 
+// Most browsers can trigger timers too early. Test data shows instances where
+// timers fire even 15ms early. We add a safety margin to prevent flakiness
+// when running this test on affected platforms.
+Duration safetyMargin = const bool.fromEnvironment('dart.library.js')
+    ? Duration(milliseconds: 40)
+    : Duration.zero;
+
 check(delayms, value) {
   Duration delay = durationInMilliseconds(delayms);
   Stopwatch sw = new Stopwatch();
   asyncStart();
   sw.start();
   new Future.delayed(delay, () {
     Duration elapsed = sw.elapsed;
-    Expect.isTrue(elapsed >= delay, "delay=$delay, elapsed=${elapsed}");
+    Expect.isTrue(elapsed + safetyMargin >= delay,
+        "delay=$delay, elapsed=${elapsed + safetyMargin}");
     asyncEnd();
   });
 }

LibTest/async/Stream/Stream.periodic_A01_t01.dart

@@ -6,33 +6,40 @@
 ///     [T computation(int computationCount)])
 /// Creates a stream that repeatedly emits events at period intervals.
 /// If computation is omitted the event values will all be null.
+///
 /// @description Checks that created stream emits events at period intervals.
 /// Checks that if computation is omitted the event value is null.
 /// @author kaigorodov
 
 import "dart:async";
 import "../../../Utils/expect.dart";
 
+// Most browsers can trigger timers too early. Test data shows instances where
+// timers fire even 15ms early. We add a safety margin to prevent flakiness
+// when running this test on affected platforms.
+Duration safetyMargin = const bool.fromEnvironment('dart.library.js')
+    ? Duration(milliseconds: 40)
+    : Duration.zero;
+
 void check(int periodMs) {
   Duration period = durationInMilliseconds(periodMs);
   const int maxCount = 5;
   int count = 0;
 
   Stopwatch sw = new Stopwatch();
   sw.start();
-
   asyncStart();
-
   Stream s = new Stream.periodic(period);
-  StreamSubscription? ss;
+  late StreamSubscription ss;
   ss = s.listen((var event) {
     count++;
     Expect.isNull(event);
     Duration expected = period * count;
     Duration actual = sw.elapsed;
-    Expect.isTrue(expected <= actual, "expected=$expected, actual=$actual");
+    Expect.isTrue(expected <= actual + safetyMargin,
+        "expected=$expected, actual=${actual + safetyMargin}");
     if (count >= maxCount) {
-      ss?.cancel();
+      ss.cancel();
       asyncEnd();
     }
   });

LibTest/async/Timer/Timer_A01_t01.dart

@@ -5,12 +5,21 @@
 /// @assertion factory Timer(Duration duration, void callback())
 /// Creates a new timer.
 /// The callback is invoked after the given duration.
-/// @description Checks that callback function is called after the given duration.
+///
+/// @description Checks that callback function is called after the given
+/// duration.
 /// @author kaigorodov
 
 import "dart:async";
 import "../../../Utils/expect.dart";
 
+// Most browsers can trigger timers too early. Test data shows instances where
+// timers fire even 15ms early. We add a safety margin to prevent flakiness
+// when running this test on affected platforms.
+Duration safetyMargin = const bool.fromEnvironment('dart.library.js')
+    ? Duration(milliseconds: 40)
+    : Duration.zero;
+
 check(int delayms) {
   Duration delay = durationInMilliseconds(delayms);
   Stopwatch sw = new Stopwatch();
@@ -19,7 +28,8 @@ check(int delayms) {
   asyncStart();
   new Timer(delay, () {
     Duration actual = sw.elapsed;
-    Expect.isTrue(delay <= actual, "expected=$delay, actual=$actual");
+    Expect.isTrue(delay <= actual + safetyMargin,
+        "expected=$delay, actual=${actual + safetyMargin}");
     asyncEnd();
   });
 }

LibTest/html/Element/onMouseOver_A01_t01.dart

@@ -4,21 +4,24 @@
 
 /// @assertion ElementStream<MouseEvent> get onMouseOver
 /// Stream of mouseover events handled by this Element.
+///
 /// @description Checks that correct events are delivered via the stream
 
 import "dart:html";
 import "../../../Utils/expect.dart";
 
 main() {
+  bool fired = false;
   var type = 'mouseover';
   var x = document.body;
   if (x != null) {
     asyncStart();
     x.onMouseOver.listen((e) {
       Expect.equals(type, e.type);
-      asyncEnd();
+      if (!fired) {
+        asyncEnd();
+      }
     });
-
     var event = new MouseEvent(type);
     x.dispatchEvent(event);
   } else {