add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/observe/lib/src/path_observer.dart

Index: pkg/observe/lib/src/path_observer.dart
diff --git a/pkg/observe/lib/src/path_observer.dart b/pkg/observe/lib/src/path_observer.dart
index ddc37e88ccdbeca0cfa49b4bd94d72bd79c8e23b..c6e0be02bc30869968f14ca8d61dbf16d5144461 100644
--- a/pkg/observe/lib/src/path_observer.dart
+++ b/pkg/observe/lib/src/path_observer.dart
@@ -62,14 +62,14 @@ class PathObserver extends ChangeNotifierBase {
   get object => _values[0];
 
   /** Gets the last reported value at this path. */
-  get value {
+  @reflectable get value {
     if (!_isValid) return null;
     if (!hasObservers) _updateValues();
     return _values.last;
   }
 
   /** Sets the value at this path. */
-  void set value(Object value) {
+  @reflectable void set value(Object value) {
     int len = _segments.length;
 
     // TODO(jmesserly): throw if property cannot be set?