Implicit index signatures

An object literal type is now assignable to a type with an index signature if all known properties in the object literal are assignable to that index signature. This makes it possible to pass a variable that was initialized with an object literal as parameter to a function that expects a map or dictionary:

function httpService(path: string, headers: { [x: string]: string }) { }
const headers = {
    "Content-Type": "application/x-www-form-urlencoded"
};
httpService("", { "Content-Type": "application/x-www-form-urlencoded" });  // Ok
httpService("", headers);  // Now ok, previously wasn't