plowshare requires /usr/bin/js. This is missing in dev-lang/spidermonkey-1.8*, the last version of that in portage that has it is 1.7.0-r2.
Maybe the javascript shell just got moved to a different location? According to spidermonkey 1.8 release notes, the javascript shell still exists (https://developer.mozilla.org/En/SpiderMonkey/1.8#New_JavaScript_shell_functions)
(In reply to comment #1) > Maybe the javascript shell just got moved to a different location? You're right - the shell gets built correctly as src/shell/js but "make install" doesn't actually install it. There's already an upstream bug for it (URL added) so I may as well close this one and wait it out.