[refactor] Switch from Array<type> notation to type[] syntactic sugar.

This is more idiomatic.
Exactly the same semantics as before, per section 3.6.4 of the [Typescript Lang Specification](http://www.typescriptlang.org/Content/TypeScript%20Language%20Specification.pdf).

I verified that it compiles (both on the test code and for [Plottable.js](https://github.com/palantir/plottable/))