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

lib/src/list_path_observer.dart

Index: lib/src/list_path_observer.dart
diff --git a/lib/src/list_path_observer.dart b/lib/src/list_path_observer.dart
index 22a81995989483ad5876c4e2328c05a4f44e3584..36764a4da00d81822d3d10eacf8fbc02de761748 100644
--- a/lib/src/list_path_observer.dart
+++ b/lib/src/list_path_observer.dart
@@ -21,9 +21,7 @@ class ListPathObserver<E, P> extends ChangeNotifier {
   bool _scheduled = false;
   Iterable<P> _value;
 
-  ListPathObserver(this.list, String path)
-      : _itemPath = path {
-
+  ListPathObserver(this.list, String path) : _itemPath = path {
     // TODO(jmesserly): delay observation until we are observed.
     _sub = list.listChanges.listen((records) {
       for (var record in records) {
@@ -46,7 +44,7 @@ class ListPathObserver<E, P> extends ChangeNotifier {
 
   void _reduce() {
     _scheduled = false;
-    var newValue = _observers.map((o) => o.value);
+    var newValue = _observers.map((o) => o.value as P);
     _value = notifyPropertyChange(#value, _value, newValue);
   }