Added a way to let the outer event loop run periodically so that things like

pkg/unittest/lib/unittest.dart

Index: pkg/unittest/lib/unittest.dart
===================================================================
--- pkg/unittest/lib/unittest.dart	(revision 25443)
+++ pkg/unittest/lib/unittest.dart	(working copy)
@@ -200,6 +200,12 @@
 final List<TestCase> testCases = new UnmodifiableListView<TestCase>(_testCases);
 
 /**
+ * Interval (in msecs) after which synchronous tests will insert an async
+ * delay to allow DOM or other updates.
+ */
+const int BREATH_INTERVAL = 200;
+
+/**
  * The set of tests to run can be restricted by using [solo_test] and
  * [solo_group].
  * As groups can be nested we use a counter to keep track of the nest level
@@ -293,6 +299,9 @@
 
 String _uncaughtErrorMessage = null;
 
+/** Time since we last gave non-sync code a chance to be scheduled. */
+int _lastBreath = new DateTime.now().millisecondsSinceEpoch;
+
 /** Test case result strings. */
 // TODO(gram) we should change these constants to use a different string
 // (so that writing 'FAIL' in the middle of a test doesn't
@@ -646,7 +655,7 @@
 void _nextTestCase() {
   runAsync(() {
     _currentTestCaseIndex++;
-    _nextBatch();
+    _nextTest();
   });
 }
 
@@ -695,7 +704,7 @@
   _config.onStart();
 
   runAsync(() {
-    _nextBatch();
+    _nextTest();
   });
 }
 
@@ -740,25 +749,23 @@
 }
 
 /**
- * Runs a batch of tests, yielding whenever an asynchronous test starts
- * running. Tests will resume executing when such asynchronous test calls
- * [done] or if it fails with an exception.
+ * Runs the next test.
  */
-void _nextBatch() {
-  while (true) {
-    if (_currentTestCaseIndex >= testCases.length) {
-      _completeTests();
-      break;
-    }
+void _nextTest() {
+  if (_currentTestCaseIndex >= testCases.length) {
+    _completeTests();
+  } else {
     final testCase = testCases[_currentTestCaseIndex];
     var f = _guardAsync(testCase._run, null, testCase);
-    if (f != null) {
-      f.whenComplete(() {
+    f.whenComplete(() {
+      var now = new DateTime.now().millisecondsSinceEpoch;
+      if ((now - _lastBreath) >= BREATH_INTERVAL) {

[Siggi Cherem (dart-lang), 2013/07/24 23:34:06]

consider moving this test up to _nextTestCase? then the question is just whether
to do runAsync or Timer.run there?

[gram, 2013/07/24 23:54:52]

_nextTestCase first increments the counter so there is a difference. I have
renamed this to _runTest which is a bit more accurate and makes the distinction
with _nextTestCase a bit clearer.

[Siggi Cherem (dart-lang), 2013/07/25 00:13:09]

basically it seems strange that we do runAsync on one case and Timer.run +
runAsync in the other.

Maybe remove runAsync from nextTestcase, but put it below in the else branch?

[gram, 2013/07/25 00:33:22]

Done.

+        _lastBreath = now;
+        new Future(_nextTestCase);

[Siggi Cherem (dart-lang), 2013/07/24 23:34:06]

isn't this the same as runAsync(_nextTestCase).

If we want to explicitly let the DOM run, we'd need to do
Timer.run(_nextTextCase)

[gram, 2013/07/24 23:54:52]

Done.

+      } else {
         _nextTestCase(); // Schedule the next test.
-      });
-      break;
-    }
-    _currentTestCaseIndex++;
+      }
+    });
   }
 }