Fix compile errors in tsbuildWatchMode.ts

[ghost]

For some reason, this compile error only appears when running gulp runtests-browser but not jake.
core is explicitly defined to have 2 or 4 elements, so accessing 3 elements is suspect -- so adding TODO comments.

[sheetalkamat]

Don't need todo. It is correct. Just ! is enough.

[weswigham]

The issue also occurs when doing a nightly build, which means it's caused by an LKG/using the built compiler probably.