Revert "Revert "Use native async/await support in pub.""

sdk/bin/pub.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.
 
 rem Run pub.dart on the Dart VM. This script is only used when running pub from
 rem within the Dart source repo. The shipped SDK instead uses "pub_sdk.bat",
 rem which is renamed to "pub.bat" when the SDK is built.
 
 setlocal
@@ skipping 11 matching lines @@
 IF %SDK_DIR:~-1%==\ set SDK_DIR=%SDK_DIR:~0,-1%
 
 set VM_OPTIONS=
 
 rem Use the Dart binary in the built SDK so pub can find the version file next
 rem to it.
 set BUILD_DIR=%SDK_DIR%\..\build\ReleaseIA32
 set PACKAGES_DIR=%BUILD_DIR%\packages
 set DART=%BUILD_DIR%\dart-sdk\bin\dart
 
-rem Run the async/await compiled pub.
-set PUB="%SDK_DIR%\lib\_internal\pub_generated\bin\pub.dart"
+rem Run pub.
+set PUB="%SDK_DIR%\lib\_internal\pub.dart"
 "%DART%" %VM_OPTIONS% --package-root="%PACKAGES_DIR%" "%PUB%" %*
 
 endlocal
 
 exit /b %errorlevel%
 
 :follow_links
 setlocal
 for %%i in (%1) do set result=%%~fi
 set current=
 for /f "usebackq 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