Support dartk on Windows.

tools/dartk_wrappers/dartk.bat

Index: tools/dartk_wrappers/dartk.bat
diff --git a/tools/dartk_wrappers/dartk.bat b/tools/dartk_wrappers/dartk.bat
new file mode 100644
index 0000000000000000000000000000000000000000..9d0efb36ab57ce1e27dd8188ec057f053f93b73d
--- /dev/null
+++ b/tools/dartk_wrappers/dartk.bat
@@ -0,0 +1,20 @@
+@echo off
+REM Copyright (c) 2017, 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%
+
+set DART_ROOT=%SCRIPTPATH%..\..\
+
+rem Remove trailing backslash if there is one
+if %DART_ROOT:~-1%==\ set DART_ROOT=%DART_ROOT:~0,-1%
+
+set DARTK=%DART_ROOT%\pkg\kernel\bin\dartk.dart
+
+set DART=%DART_ROOT%\tools\sdks\win\dart-sdk\bin\dart
+
+"%DART%" --packages=.packages "%DARTK%" %*