add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/observe/lib/src/compound_binding.dart

Index: pkg/observe/lib/src/compound_binding.dart
diff --git a/pkg/observe/lib/src/compound_binding.dart b/pkg/observe/lib/src/compound_binding.dart
index 4e65274dc49c8d0b0ad6885a502e475d4dcc9a08..a6195864524fd1fb39e8b00e6f543417332084cb 100644
--- a/pkg/observe/lib/src/compound_binding.dart
+++ b/pkg/observe/lib/src/compound_binding.dart
@@ -65,11 +65,11 @@ class CompoundBinding extends ChangeNotifierBase {
     if (combinator != null) _scheduleResolve();
   }
 
-  get value => _value;
-
   int get length => _observers.length;
 
-  void set value(newValue) {
+  @reflectable get value => _value;
+
+  @reflectable void set value(newValue) {
     _value = notifyPropertyChange(#value, _value, newValue);
   }