add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer_expressions/lib/src/globals.dart

Index: pkg/polymer_expressions/lib/src/globals.dart
diff --git a/pkg/polymer_expressions/lib/src/globals.dart b/pkg/polymer_expressions/lib/src/globals.dart
index b51aeb3adbfc53e78ac112757e7cf0bacfb246b8..281536df329557d2b7ecedbaf35c01262021bdd1 100644
--- a/pkg/polymer_expressions/lib/src/globals.dart
+++ b/pkg/polymer_expressions/lib/src/globals.dart
@@ -11,6 +11,7 @@
 library polymer_expressions.src.globals;
 
 import 'dart:collection';
+import 'package:observe/observe.dart' show reflectable;
 
 /**
  * Returns an [Iterable] of [IndexedValue]s where the nth value holds the nth
@@ -19,7 +20,7 @@ import 'dart:collection';
 Iterable<IndexedValue> enumerate(Iterable iterable) =>
     new EnumerateIterable(iterable);
 
-class IndexedValue<V> {
+@reflectable class IndexedValue<V> {
   final int index;
   final V value;