Attempting to remove flakiness from history test

tests/html/history_test.dart

Index: tests/html/history_test.dart
diff --git a/tests/html/history_test.dart b/tests/html/history_test.dart
index 462acb373a1c55ef1194a5a0e49725c3a5eb8def..bee3323b8866b1d7bc8748e877fb9b5fdea94e7c 100644
--- a/tests/html/history_test.dart
+++ b/tests/html/history_test.dart
@@ -45,12 +45,15 @@ main() {
 
         expect(window.location.href.endsWith('dummy2'), isTrue);
 
-        expectAsync1Once(window.on.popState, (_) {
-          expect(window.history.length, length);
-          expect(window.location.href.endsWith('dummy1'), isTrue);
-        });
+        // Need to wait a frame or two to let the pushState events occur.
+        window.setTimeout(expectAsync0(() {
+          expectAsync1Once(window.on.popState, (_) {
+            expect(window.history.length, length);
+            expect(window.location.href.endsWith('dummy1'), isTrue);
+          });
 
-        window.history.back();
+          window.history.back();
+        }), 100);
       }, expectation);
     });