Add a test for #833

This test just exercises Align to make sure it can sync.

packages/unit/test/widget/align_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'build_utils.dart';
+
+void main() {
+  test('Global key mutate during iteration', () {
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
+      return new Align(
+        child: new Container(),
+        horizontal: 0.75,
+        vertical: 0.75
+      );
+    });
+
+    tester.pumpFrame(() {
+      return new Align(
+        child: new Container(),
+        horizontal: 0.5,
+        vertical: 0.5
+      );
+    });
+
+  });
+}