Enable checker.py to execute type-checking almost all of the JavaScript files at once

This CL enables checker.py to execute type-checking almost all of the JavaScript files at once. We can execute this just with `checker.cmd` or `checker.sh` without any arguments.

I eliminated testing/*.js and polyfill.js from the set of the target because they cause a lot of warnings which seem  difficult to solve.

This execution takes much less time than when we execute every test separately.

PTAL