fix observe package to be strong mode clean

lib/src/bind_property.dart

Index: lib/src/bind_property.dart
diff --git a/lib/src/bind_property.dart b/lib/src/bind_property.dart
index 805507a94c5101b043c114b2e76234318c4f27d3..202859d0014200dadfeaa6a968d5d14dbcebd0e1 100644
--- a/lib/src/bind_property.dart
+++ b/lib/src/bind_property.dart
@@ -29,8 +29,7 @@ StreamSubscription onPropertyChange(
     Observable source, Symbol sourceName, void callback()) {
   return source.changes.listen((records) {
     for (var record in records) {
-      if (record is PropertyChangeRecord &&
-          (record as PropertyChangeRecord).name == sourceName) {
+      if (record is PropertyChangeRecord && record.name == sourceName) {
         callback();
         break;
       }