Clean up some Dart idioms in fn

sky/examples/fn/lib/reflect.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 library reflect;
 
 import 'dart:mirrors';
 import 'dart:collection';
 
-HashMap<ClassMirror, List> _fieldCache = new HashMap<ClassMirror, List>();
+final HashMap<ClassMirror, List> _fieldCache = new HashMap<ClassMirror, List>();
 
 List<Symbol> _getPublicFields(ClassMirror mirror) {
-  var fields = _fieldCache[mirror];
-  if (fields == null) {
-    fields = new List<Symbol>();
-    _fieldCache[mirror] = fields;
+  return _fieldCache.putIfAbsent(mirror, () {
+    List<Symbol> fields = new List<Symbol>();
 
     while (mirror != null) {
       var decls = mirror.declarations;
       fields.addAll(decls.keys.where((symbol) {
         var mirror = decls[symbol];
         if (mirror is! VariableMirror) {
           return false;
         }
 
         var vMirror = mirror as VariableMirror;
         return !vMirror.isPrivate && !vMirror.isStatic && !vMirror.isFinal;
       }));
 
       mirror = mirror.superclass;
     }
-  }
 
-  return fields;
+    return fields;
+  });
 }
 
 void copyPublicFields(Object source, Object target) {
   assert(source.runtimeType == target.runtimeType);
 
   var sourceMirror = reflect(source);
   var targetMirror = reflect(target);
   for (var symbol in _getPublicFields(sourceMirror.type)) {
     targetMirror.setField(symbol, sourceMirror.getField(symbol).reflectee);
   }
 }