To repro, run:

   USE=asan FEATURES=test emerge-coral modemmanager-next  # any amd64 board works too


 * ERROR: net-misc/modemmanager-next-9999::chromiumos failed (test phase):
 *   asan error not caught
 * 
 * Call stack:
 *        ebuild.sh, line  133:  Called post_src_test
 *      environment, line 4438:  Called cros_stack_hooks 'post_src_test'
 *   profile.bashrc, line   86:  Called cros_post_src_test_asan_check
 *   profile.bashrc, line  209:  Called die
 * The specific snippet of code:
 *      rmdir "${T}/asan_logs" 2>/dev/null || die "asan error not caught"
 * 
 * If you need support, post the output of `emerge --info '=net-misc/modemmanager-next-9999::chromiumos'`,
 * the complete build log and the output of `emerge -pqv '=net-misc/modemmanager-next-9999::chromiumos'`.
 * ASAN error detected:
 * ==18803==WARNING: LeakSanitizer is disabled in forked process.
 * ASAN error detected:
 * ==18805==WARNING: LeakSanitizer is disabled in forked process.
 * ASAN error detected:
 * ==18807==WARNING: LeakSanitizer is disabled in forked process.
 * ASAN error detected:
 * ==18809==WARNING: LeakSanitizer is disabled in forked process.
 * ASAN error detected:
 * ==18940==WARNING: LeakSanitizer is disabled in forked process.
 * The complete build log is located at '/build/coral/tmp/portage/logs/net-misc:modemmanager-next-9999:20181005-175738.log'.
 * For convenience, a symlink to the build log is located at '/build/coral/tmp/portage/net-misc/modemmanager-next-9999/temp/build.log'.
 * The ebuild environment file is located at '/build/coral/tmp/portage/net-misc/modemmanager-next-9999/temp/environment'.
 * Working directory: '/build/coral/tmp/portage/net-misc/modemmanager-next-9999/work/modemmanager-next-9999'
 * S: '/build/coral/tmp/portage/net-misc/modemmanager-next-9999/work/modemmanager-next-9999'
make: *** [/home/benchan/shared/scripts/cros.mk:68: t/modemmanager-next] Error 1

It looks like asan_logs contains a few asan logs that only show a warning "LeakSanitizer is disabled in forked process.", which triggers the following check:

# Check for any ASAN failures that were missed while testing.
cros_post_src_test_asan_check() {
	rmdir "${T}/asan_logs" 2>/dev/null || die "asan error not caught"
	mkdir -p "${T}/asan_logs"
}