remove docgen source and targets from build

sdk/bin/dartdocgen.bat

-@echo off
-REM Copyright (c) 2014, 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 DOCGEN=%SDK_DIR%\pkg\docgen\bin\docgen.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%" "docgen" "--sdk=%SDK_DIR%" %*
-) else (
-  "%BUILD_DIR%\dart-sdk\bin\dart" "--package-root=%BUILD_DIR%\packages" "%DOCGEN%" "--sdk=%SDK_DIR%" %*
-)
-
-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