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..ba9d1041ac4c010456ccd1e9cc7aff642f7fcf50 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) {
@@ -47,7 +45,7 @@ class ListPathObserver<E, P> extends ChangeNotifier {
   void _reduce() {
     _scheduled = false;
     var newValue = _observers.map((o) => o.value);
-    _value = notifyPropertyChange(#value, _value, newValue);
+    _value = notifyPropertyChange(#value, _value, newValue) as Iterable<P>;

[vsm, 2016/01/22 16:47:15]

Perhaps we need to be more permissive on casts from dynamic.  Or provide a
static cast?

[Jennifer Messerly, 2016/01/22 17:45:00]

generic methods FTW!

    notifyPropertyChange<T>(Symbol name, T oldValue, T newValue)

The problem is, I'm not sure typing is right on _observers. I think it should be
PathObserver<P>, but not sure PathObserver is generic.

The easy fix here though is probably this:

    var newValue = _observers.map((o) => o.value as P);
    notifyPropertyChange(#value, _value, newValue);
    _value = newValue;

[vsm, 2016/01/22 18:49:22]

Done.

   }
 
   void _scheduleReduce(_) {