Add a snapshot for docgen and use it in the build

sdk/bin/dartdoc.bat

 @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.
 
 setlocal
 rem Handle the case where dart-sdk/bin has been symlinked to.
 set DIR_NAME_WITH_SLASH=%~dp0
 set DIR_NAME=%DIR_NAME_WITH_SLASH:~0,-1%%
 call :follow_links "%DIR_NAME%", RETURNED_BIN_DIR
 rem Get rid of surrounding quotes.
 for %%i in ("%RETURNED_BIN_DIR%") do set BIN_DIR=%%~fi
 
 rem Get absolute full name for SDK_DIR.
 for %%i in ("%BIN_DIR%\..\") do set SDK_DIR=%%~fi
 
 rem Remove trailing backslash if there is one
 IF %SDK_DIR:~-1%==\ set SDK_DIR=%SDK_DIR:~0,-1%
 
 set DARTDOC=%SDK_DIR%\lib\_internal\dartdoc\bin\dartdoc.dart
 set DART=%BIN_DIR%\dart
 set SNAPSHOT=%BIN_DIR%\snapshots\utils_wrapper.dart.snapshot
 
 if not defined DART_CONFIGURATION set DART_CONFIGURATION=ReleaseIA32
 
 set BUILD_DIR=%SDK_DIR%\..\build\%DART_CONFIGURATION%
 if exist "%SNAPSHOT%" (
   "%DART%" "%SNAPSHOT%" "dartdoc" "--library-root=%SDK_DIR%" %*
 ) else (
-  :: The trailing forward slash in --package-root is required because of issue
-  :: 9499.
-  "%BUILD_DIR%\dart-sdk\bin\dart" "--package-root=%BUILD_DIR%\packages/" "%DARTDOC%" %*
+  "%BUILD_DIR%\dart-sdk\bin\dart" "--package-root=%BUILD_DIR%\packages" "%DARTDOC%" %*
 )
 
 endlocal
 
 exit /b %errorlevel%
 
 :follow_links
 setlocal
 for %%i in (%1) do set result=%%~fi
 set current=
 for /f "tokens=2 delims=[]" %%i in ('dir /a:l ^"%~dp1^" 2^>nul ^
                                      ^| find ">     %~n1 ["') do (
   set current=%%i
 )
 if not "%current%"=="" call :follow_links "%current%", result
 endlocal & set %~2=%result%
 goto :eof
 
 :end