Clean up some Dart idioms in fn

sky/examples/fn/lib/fn.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 fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 import 'reflect.dart' as reflect;
 
 part 'component.dart';
 part 'node.dart';
 part 'style.dart';
 
 bool _checkedMode;
 
-bool debugWarnings() {
+bool _debugWarnings() {
   void testFn(double i) {}
 
   if (_checkedMode == null) {
     _checkedMode = false;
     try {
       testFn('not a double');
     } catch (ex) {
       _checkedMode = true;
     }
   }
 
   return _checkedMode;
 }