Clean up some Dart idioms in fn

sky/examples/fn/lib/style.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.
+
 part of fn;
 
 class Style {
   final String _className;
-  static Map<String, Style> _cache = null;
+  static final Map<String, Style> _cache = new HashMap<String, Style>();
 
   static int nextStyleId = 1;
 
   static String nextClassName(String styles) {
     assert(sky.document != null);
-    var className = "style$nextStyleId";
+    String className = "style$nextStyleId";
     nextStyleId++;
 
-    var styleNode = sky.document.createElement('style');
+    sky.Element styleNode = sky.document.createElement('style');
     styleNode.setChild(new sky.Text(".$className { $styles }"));
     sky.document.appendChild(styleNode);
 
     return className;
   }
 
   factory Style(String styles) {
-    if (_cache == null) {
-      _cache = new HashMap<String, Style>();
-    }
-
-    var style = _cache[styles];
-    if (style == null) {
-      style = new Style._internal(nextClassName(styles));
-      _cache[styles] = style;
-    }
-
-    return style;
+    return _cache.putIfAbsent(styles, () {
+      return new Style._internal(nextClassName(styles));
+    });
   }
 
   Style._internal(this._className);
 }