TestCase times out after 20 seconds.

dart/pkg/unittest/lib/unittest.dart

Index: dart/pkg/unittest/lib/unittest.dart
diff --git a/dart/pkg/unittest/lib/unittest.dart b/dart/pkg/unittest/lib/unittest.dart
index eb186bd51716d679f21bd52fc75eb9f79a17b6fb..9aa514ad27bf1d24c2435f5f4bb4fbd53cdd90e8 100644
--- a/dart/pkg/unittest/lib/unittest.dart
+++ b/dart/pkg/unittest/lib/unittest.dart
@@ -752,7 +752,18 @@ void _runTest() {
   } else {
     final testCase = testCases[_currentTestCaseIndex];
     var f = _guardAsync(testCase._run, null, testCase);
+    Timer timer;
+    try {
+      final Duration timeout = unittestConfiguration.timeout;
+      timer = new Timer(timeout, () {
+        testCase.error("Test timed out after ${timeout.inSeconds} seconds.");
+      });
+    } on UnsupportedError catch (e) {
+      if (e.message != "Timer greater than 0.") rethrow;
+      // Support running on d8 and jsshell which don't support timers.
+    }
     f.whenComplete(() {
+      if (timer != null) timer.cancel();
       var now = new DateTime.now().millisecondsSinceEpoch;
       if ((now - _lastBreath) >= BREATH_INTERVAL) {
         _lastBreath = now;