i am getting error with on that can u please help on that

Unknown argument: -Xss

Google Web Toolkit 2.5.1

Compiler [-logLevel level] [-workDir dir] [-gen dir] [-style style] [-ea] [-XdisableClassMetadata] [-XdisableCastChecking] [-validateOnly] [-draftCompile] [-optimize level] [-compileReport] [-strict] [-XenableClosureCompiler] [-XfragmentMerge numFragments] [-XfragmentCount numFragments] [-localWorkers count] [-war dir] [-deploy dir] [-extra dir] module[s] 

where 

  -logLevel                The level of logging detail: ERROR, WARN, INFO, TRACE, DEBUG, SPAM, or ALL

  -workDir                 The compiler's working directory for internal use (must be writeable; defaults to a system temp dir)

  -gen                     Debugging: causes normally-transient generated types to be saved in the specified directory

  -style                   Script output style: OBF[USCATED], PRETTY, or DETAILED (defaults to OBF)

  -ea                      Debugging: causes the compiled output to check assert statements

  -XdisableClassMetadata   EXPERIMENTAL: Disables some java.lang.Class methods (e.g. getName())

  -XdisableCastChecking    EXPERIMENTAL: Disables run-time checking of cast operations

  -validateOnly            Validate all source code, but do not compile

  -draftCompile            Enable faster, but less-optimized, compilations

  -optimize                Sets the optimization level used by the compiler.  0=none 9=maximum.

  -compileReport           Create a compile report that tells the Story of Your Compile

  -strict                  Only succeed if no input files have errors

  -XenableClosureCompiler  EXPERIMENTAL: Enables Closure Compiler optimizations

  -XfragmentMerge          DEPRECATED (use -XfragmentCount instead): Enables Fragment merging code splitter.

  -XfragmentCount          EXPERIMENTAL: Limits of number of fragments using a code splitter that merges split points.

  -localWorkers            The number of local workers to use when compiling permutations

  -war                     The directory into which deployable output files will be written (defaults to 'war')

  -deploy                  The directory into which deployable but not servable output files will be written (defaults to 'WEB-INF/deploy' under the -war directory/jar, and may be the same as the -extra directory/jar)

  -extra                   The directory into which extra files, not intended for deployment, will be written

and 

  module[s]                Specifies the name(s) of the module(s) to compile

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