`gulp runtests-browser` broken

TypeScript Version: master

• In host.ts, require("mocha") returns the global object. (There's a line module.exports = global; in mocha.js, so apparently this is intended.) You probably meant require("mocha").Mocha.
• I'm using a valid regexp, but it doesn't seem to run any tests.

Reverting to before #18462 fixes the problem. CC @weswigham