Fixed strong mode errors and warnings reachable from lib/observe.dart

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 21e14f96e86389775b31404ff0df23425e9cb186..805507a94c5101b043c114b2e76234318c4f27d3 100644
--- a/lib/src/bind_property.dart
+++ b/lib/src/bind_property.dart
@@ -25,8 +25,8 @@ import 'package:observe/observe.dart';
 ///
 /// See also [notifyPropertyChange].
 // TODO(jmesserly): make this an instance method?
-StreamSubscription onPropertyChange(Observable source, Symbol sourceName,
-    void callback()) {
+StreamSubscription onPropertyChange(
+    Observable source, Symbol sourceName, void callback()) {
   return source.changes.listen((records) {
     for (var record in records) {
       if (record is PropertyChangeRecord &&