UserAgent: Mozilla/5.0 (X11; CrOS x86_64 7978.74.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.103 Safari/537.36

Steps to reproduce the problem:
1. dev_install -y

What is the expected behavior?
Install dev_install properly

What went wrong?
 * Messages for package sys-devel/gdb-7.7.1-r4 merged to /usr/local/:

 * ERROR: sys-devel/gdb-7.7.1-r4::chromiumos failed (postinst phase):
 *   USE Flag 'prefix' not in IUSE for sys-devel/gdb-7.7.1-r4
 * 
 * Call stack:
 *          ebuild.sh, line   94:  Called pkg_postinst
 *        environment, line 2751:  Called use 'prefix'
 *   phase-helpers.sh, line  200:  Called die
 * The specific snippet of code:
 *                              die "USE Flag '${u}' not in IUSE for ${CATEGORY}/${PF}"
 * 
 * If you need support, post the output of `emerge --info '=sys-devel/gdb-7.7.1-r4::chromiumos'`,
 * the complete build log and the output of `emerge -pqv '=sys-devel/gdb-7.7.1-r4::chromiumos'`.
 * The complete build log is located at '/usr/local/var/tmp/portage/sys-devel/gdb-7.7.1-r4/temp/build.log'.
 * The ebuild environment file is located at '/usr/local/var/tmp/portage/sys-devel/gdb-7.7.1-r4/temp/environment'.
 * Working directory: '/usr/local/lib/portage/pym'
 * S: '/usr/local/var/tmp/portage/sys-devel/gdb-7.7.1-r4/work/gdb-7.7.1'

Did this work before? No 

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