Emit rest parameters natively in ES6

Why not just have

```TypeScript
if (hasRestParam) {
    emitCommaList(node.parameters, /*includeTrailingComma*/ false, numberOfParameters - (hasRestParam ? 1 : 0));
}
else {
    emitCommaList(node.parameters, /*includeTrailingComma*/ false, numberOfParameters);
}
```

and edit `emitParameter` to be

```TypeScript
function emitParameter(node: ParameterDeclaration) {
    emitLeadingComments(node);
    if (node.flags & NodeFlags.Rest) {
        write("...");
    }
    emit(node.name);
    emitTrailingComments(node);
}
```