Add safetymargin for timer tests in browsers.

tests/lib/async/timer_test.dart

Index: tests/lib/async/timer_test.dart
diff --git a/tests/lib/async/timer_test.dart b/tests/lib/async/timer_test.dart
index 89b38ece5d7adbd847acca084a7aa84356c5bf3a..327dc18a0b7bf33cb0d9422ed7d4033b3e5d8830 100644
--- a/tests/lib/async/timer_test.dart
+++ b/tests/lib/async/timer_test.dart
@@ -15,9 +15,14 @@ int startTime;
 int timeout;
 int iteration;
 
+// 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) ? 0 : 100;
+
 void timeoutHandler() {
   int endTime = (new DateTime.now()).millisecondsSinceEpoch;
-  expect(endTime - startTime, greaterThanOrEqualTo(timeout));
+  expect(endTime - startTime + safetyMargin, greaterThanOrEqualTo(timeout));
   if (iteration < ITERATIONS) {
     iteration++;
     timeout = timeout - DECREASE;