Add a new Theme widget to control color and text color of apps

sky/sdk/lib/theme/typography.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.
 
 // See http://www.google.com/design/spec/style/typography.html
 
 import 'dart:sky';
 
 import '../painting/text_style.dart';
 
 // TODO(eseidel): Font weights are supposed to be language relative!
 // These values are for English-like text.
-class _TextTheme {
-  _TextTheme(Color color54, Color color87)
+class TextTheme {
+  TextTheme._(Color color54, Color color87)
     : display4 = new TextStyle(fontSize: 112.0, fontWeight: FontWeight.w100, color: color54),
       display3 = new TextStyle(fontSize:  56.0, fontWeight: FontWeight.w400, color: color54),
       display2 = new TextStyle(fontSize:  45.0, fontWeight: FontWeight.w400, color: color54),
       display1 = new TextStyle(fontSize:  34.0, fontWeight: FontWeight.w400, color: color54),
       headline = new TextStyle(fontSize:  24.0, fontWeight: FontWeight.w400, color: color87),
       title    = new TextStyle(fontSize:  20.0, fontWeight: FontWeight.w500, color: color87),
       subhead  = new TextStyle(fontSize:  16.0, fontWeight: FontWeight.w400, color: color87),
       body2    = new TextStyle(fontSize:  14.0, fontWeight: FontWeight.w500, color: color87),
       body1    = new TextStyle(fontSize:  14.0, fontWeight: FontWeight.w400, color: color87),
       caption  = new TextStyle(fontSize:  12.0, fontWeight: FontWeight.w400, color: color54),
       button   = new TextStyle(fontSize:  14.0, fontWeight: FontWeight.w500, color: color87);
 
   final TextStyle display4;
   final TextStyle display3;
   final TextStyle display2;
   final TextStyle display1;
   final TextStyle headline;
   final TextStyle title;
   final TextStyle subhead;
   final TextStyle body2;
   final TextStyle body1;
   final TextStyle caption;
   final TextStyle button;
 }
 
 
-final _TextTheme black = new _TextTheme(
+final TextTheme black = new TextTheme._(
   const Color(0xFF757575),
   const Color(0xFF212121)
 );
 
 
-final _TextTheme white = new _TextTheme(
+final TextTheme white = new TextTheme._(
   const Color(0xFF8A8A8A),
   const Color(0xFFDEDEDE)
 );
 
 // TODO(abarth): Maybe this should be hard-coded in Scaffold?
 const String typeface = 'font-family: sans-serif';