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..d82f2266ca93c6710ec7c002870519834cf2e9d8 100644
--- a/tests/lib/async/timer_test.dart
+++ b/tests/lib/async/timer_test.dart
@@ -15,9 +15,15 @@ int startTime;
 int timeout;
 int iteration;
 
+// Easy way to know if the test is compiled by dart2js.
+bool get isCompiledByDart2Js => identical(1, 1.0);
+
 void timeoutHandler() {
   int endTime = (new DateTime.now()).millisecondsSinceEpoch;
-  expect(endTime - startTime, greaterThanOrEqualTo(timeout));
+  // Some browsers (Firefox and IE so far) can trigger too early. Add a safety
+  // margin.
+  int safetyMargin = isCompiledByDart2Js ? 100 : 0;
+  expect(endTime - startTime + safetyMargin, greaterThanOrEqualTo(timeout));
   if (iteration < ITERATIONS) {
     iteration++;
     timeout = timeout - DECREASE;