console: allow Object.prototype fields as labels

Console.prototype.timeEnd() returns NaN if the timer label
corresponds to a property on Object.prototype. This commit
uses a Map to construct the _times object.

Fixes: https://github.com/joyent/node/issues/9069
PR-URL: https://github.com/iojs/io.js/pull/563


Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>