use symbol literals instead of const ctor in packages

pkg/polymer_expressions/example/person.dart

Index: pkg/polymer_expressions/example/person.dart
diff --git a/pkg/polymer_expressions/example/person.dart b/pkg/polymer_expressions/example/person.dart
index d561fac4b0fa01b7d198ca1031c2e373f948c0ff..1c591b767290dc43858ac97bed6383d863659b30 100644
--- a/pkg/polymer_expressions/example/person.dart
+++ b/pkg/polymer_expressions/example/person.dart
@@ -7,12 +7,7 @@ library person;
 import 'package:observe/observe.dart';
 
 class Person extends ChangeNotifierBase {
-  static const _FIRST_NAME = const Symbol('firstName');
-  static const _LAST_NAME = const Symbol('lastName');
-  static const _ITEMS = const Symbol('items');
-  static const _GET_FULL_NAME = const Symbol('getFullName');
-
-  String _firstName;
+\  String _firstName;
   String _lastName;
   List<String> _items;
 
@@ -22,14 +17,14 @@ class Person extends ChangeNotifierBase {
 
   void set firstName(String value) {
     _firstName = value;
-    notifyChange(new PropertyChangeRecord(_FIRST_NAME));
+    notifyChange(new PropertyChangeRecord(#firstName));
   }
 
   String get lastName => _lastName;
 
   void set lastName(String value) {
     _lastName = value;
-    notifyChange(new PropertyChangeRecord(_LAST_NAME));
+    notifyChange(new PropertyChangeRecord(#lastName));
   }
 
   String getFullName() => '$_firstName $_lastName';
@@ -38,7 +33,7 @@ class Person extends ChangeNotifierBase {
 
   void set items(List<String> value) {
     _items = value;
-    notifyChange(new PropertyChangeRecord(_ITEMS));
+    notifyChange(new PropertyChangeRecord(#items));
   }
 
   String toString() => "Person(firstName: $_firstName, lastName: $_lastName)";