add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/observe/lib/src/list_path_observer.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of observe;
 
 // Inspired by ArrayReduction at:
 // https://raw.github.com/rafaelw/ChangeSummary/master/util/array_reduction.js
 // The main difference is we support anything on the rich Dart Iterable API.
 
@@ skipping 18 matching lines @@
           _observeItems(record.addedCount - record.removedCount);
         }
       }
       _scheduleReduce(null);
     });
 
     _observeItems(list.length);
     _reduce();
   }
 
-  Iterable<P> get value => _value;
+  @reflectable Iterable<P> get value => _value;
 
   void dispose() {
     if (_sub != null) _sub.cancel();
     _subs.forEach((s) => s.cancel());
     _subs.clear();
   }
 
   void _reduce() {
     _scheduled = false;
     _value = _observers.map((o) => o.value);
@@ skipping 16 matching lines @@
       }
     } else if (lengthAdjust < 0) {
       for (int i = 0; i < -lengthAdjust; i++) {
         _subs.removeLast().cancel();
       }
       int len = _observers.length;
       _observers.removeRange(len + lengthAdjust, len);
     }
   }
 }