add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer_expressions/lib/polymer_expressions.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.
 
 /**
  * A binding delegate used with Polymer elements that
  * allows for complex binding expressions, including
  * property access, function invocation,
  * list/map indexing, and two-way filtering.
  *
@@ skipping 109 matching lines @@
         vars[v.identifier] = i;
         Scope childScope = new Scope(parent: _scope, variables: vars);
         return childScope;
       }).toList(growable: false);
     } else {
       _value = (_converter == null) ? v : _converter(v);
     }
     notifyPropertyChange(#value, oldValue, _value);
   }
 
-  get value => _value;
+  @reflectable get value => _value;
 
-  set value(v) {
+  @reflectable set value(v) {
     try {
       assign(_expr, v, _scope);
     } on EvalException catch (e) {
       // silently swallow binding errors
     }
   }
 }