Add support for passing in vm options to the dart2js scripts.

sdk/bin/dart2js.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
@@ skipping 17 matching lines @@
   set EXTRA_VM_OPTIONS=%EXTRA_VM_OPTIONS% --checked
 )
 
 if exist "%SNAPSHOT%" (
   set EXTRA_OPTIONS=%EXTRA_OPTIONS% "--library-root=%SDK_DIR%"
 )
 
 rem See comments regarding options below in dart2js shell script.
 set EXTRA_VM_OPTIONS=%EXTRA_VM_OPTIONS% --heap_growth_rate=512
 
+rem We allow extra vm options to be passed in through an environment variable.
+if not _%DART_VM_OPTIONS%_ == __ (
+  set EXTRA_VM_OPTIONS=%EXTRA_VM_OPTIONS% %DART_VM_OPTIONS%
+)
+
 if exist "%SNAPSHOT%" (
   "%DART%" %EXTRA_VM_OPTIONS% "%SNAPSHOT%" %EXTRA_OPTIONS% %*
 ) else (
   "%DART%" %EXTRA_VM_OPTIONS% "%DART2JS%" %EXTRA_OPTIONS% %*
 )
 
 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