Enabled use of snapshot for dart2js on Windows.

utils/compiler/build_helper.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #import('dart:io');
 #import('dart:uri');
 
 #import('../../lib/compiler/implementation/util/uri_extras.dart');
 #import('../../lib/compiler/implementation/filenames.dart');
 
@@ skipping 124 matching lines @@
 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%
 
 set arguments=%*
+set SNAPSHOTNAME=%SCRIPTPATH%dart2js.snapshot
+if exist %SNAPSHOTNAME% set SNAPSHOT=--use_script_snapshot=%SNAPSHOTNAME%
 
-"%SCRIPTPATH%\dart.exe"$options "%SCRIPTPATH%$pathWin" %arguments%
+"%SCRIPTPATH%\dart.exe"$options %SNAPSHOT% "%SCRIPTPATH%$pathWin" %arguments%
 '''.replaceAll('\n', '\r\n')];
 }