Add safety margin to timer_isolate_test.dart

tests/lib/async/timer_isolate_test.dart

Index: tests/lib/async/timer_isolate_test.dart
diff --git a/tests/lib/async/timer_isolate_test.dart b/tests/lib/async/timer_isolate_test.dart
index 31776c5f5edb009b82cdd31b7ffd5559ed05a0b0..8c6c5fbf7165654b36853f311e6a35f8a7f6dada 100644
--- a/tests/lib/async/timer_isolate_test.dart
+++ b/tests/lib/async/timer_isolate_test.dart
@@ -10,6 +10,11 @@ import '../../../pkg/unittest/lib/unittest.dart';
 
 const Duration TIMEOUT = const Duration(milliseconds: 100);
 
+// Some browsers (Firefox and IE so far) can trigger too early. Add a safety
+// margin. We use identical(1, 1.0) as an easy way to know if the test is
+// compiled by dart2js.
+int get safetyMargin => identical(1, 1.0) ? 100 : 1;
+
 createTimer(replyTo) {
   new Timer(TIMEOUT, () {
     replyTo.send("timer_fired");
@@ -26,7 +31,7 @@ main() {
     port.first.then(expectAsync1((msg) {
       expect("timer_fired", msg);
       int endTime = (new DateTime.now()).millisecondsSinceEpoch;
-      expect(endTime - startTime, greaterThanOrEqualTo(TIMEOUT.inMilliseconds));
+      expect(endTime - startTime + safetyMargin, greaterThanOrEqualTo(TIMEOUT.inMilliseconds));
     }));
 
     startTime = (new DateTime.now()).millisecondsSinceEpoch;