Module identifiers allow for .js extension

Before TypeScript 2.0, a module identifier was always assumed to be extension-less;for instance, given an import as import d from "./moduleA.js", the compiler looked up the definition of "moduleA.js" in ./moduleA.js.ts or ./moduleA.js.d.ts.This made it hard to use bundling/loading tools like SystemJS that expect URI’s in their module identifier.

With TypeScript 2.0, the compiler will look up definition of "moduleA.js" in ./moduleA.ts or ./moduleA.d.ts.