Add windows scripts equivalent to the macos/linux scripts in sdk/bin.

sdk/bin/dart.bat

Index: sdk/bin/dart.bat
===================================================================
--- sdk/bin/dart.bat	(revision 0)
+++ sdk/bin/dart.bat	(revision 0)
@@ -0,0 +1,16 @@
+@echo off
+REM Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+REM for details. All rights reserved. Use of this source code is governed by a
+REM BSD-style license that can be found in the LICENSE file.
+
+set SCRIPTPATH=%~dp0
+
+REM Does the path have a trailing slash? If so, remove it.
+if %SCRIPTPATH:~-1%== set SCRIPTPATH=%SCRIPTPATH:~0,-1%
+
+REM DART_CONFIGURATION defaults to ReleaseIA32
+if "%DART_CONFIGURATION%"=="" set DART_CONFIGURATION=ReleaseIA32
+
+set arguments=%*
+
+"%SCRIPTPATH%\..\..\build\%DART_CONFIGURATION%\dart.exe" %arguments%