Self-execute all closures passed to assert.

In Dart 2.0, assert will only accept booleans, not closures any more. The solution to upgrade is very simple: we just self-execute any closures passed to assert today.

PiperOrigin-RevId: 182406596

Author: srawlins

lib/model/observable/observable.dart

@@ -332,7 +332,7 @@ class ObservableComposite extends ChangeNotificationProvider {
         stackTrace = StackTrace.current;
       }
       return true;
-    });
+    }());
     _subscriptions[stream] = stream.listen((_) {
       if (stackTrace != null) {
         print("Coalescer notified from $stackTrace");