Enabled use of snapshot for dart2js on Windows.

utils/compiler/build_helper.dart

Index: utils/compiler/build_helper.dart
diff --git a/utils/compiler/build_helper.dart b/utils/compiler/build_helper.dart
index 0b4d29eca59e43fae3c312e51de7a984de86fca3..971bb586c954d0aaa22d0b800a7d0c3b95b6dbd5 100644
--- a/utils/compiler/build_helper.dart
+++ b/utils/compiler/build_helper.dart
@@ -142,7 +142,9 @@ 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')];
 }