fix dart2js warnings in polymer_expressions pkg

pkg/polymer_expressions/lib/eval.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.
 
 library polymer_expressions.eval;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 
@@ skipping 590 matching lines @@
 }
 
 _toBool(v) => (v == null) ? false : v;
 
 /** Call a [Function] or a [Method]. */
 // TODO(jmesserly): remove this once dartbug.com/13002 is fixed.
 // Just inline `_convert(Function.apply(...))` to the call site.
 Object call(Object receiver, List args) {
   var result;
   if (receiver is Method) {
-    result = receiver.mirror.invoke(receiver.symbol, args, null).reflectee;
+    Method method = receiver;
+    result = method.mirror.invoke(method.symbol, args, null).reflectee;
   } else {
     result = Function.apply(receiver, args, null);
   }
   return _convert(result);
 }
 
 /**
  * A comprehension declaration ("a in b"). [identifier] is the loop variable
  * that's added to the scope during iteration. [iterable] is the set of
  * objects to iterate over.
@@ skipping 18 matching lines @@
    * This does not work for calls that need to pass more than one argument.
    */
   call(arg0) => mirror.invoke(symbol, [arg0], null).reflectee;
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }