Fix build break:

Fix build break when compiling with gcc 4.6:

../runtime/bin/file.cc: In function 'void Builtin_File_Flush(Dart_NativeArguments)':
../runtime/bin/file.cc:356:12: error: variable 'return_value' set but not used [-Werror=unused-but-set-variable]
cc1plus: all warnings being treated as errors
Committed: https://code.google.com/p/dart/source/detail?r=5923

[cshapiro, 2012-03-28 02:07:37]

lgtm

[Søren Gjesse, 2012-03-28 06:23:03]

lgtm