Make it possible to iterate over `arguments` in a for/of if it's passed to another function

[MatrixFrog]

As documented at https://github.com/google/closure-compiler/wiki/Lint-warning-about-%60arguments%60 we can't transpile the "arguments" object correctly if you iterate over it in a for/of loop, after passing it off to another function. This is because we weren't sure of a good way to identify that it is an Arguments object at runtime. @concavelenz pointed out that we can use Object.prototype.toString.call( item ) === '[object Arguments]'; which feels slightly hacky to me, but will work pretty much everywhere (I think -- we should double check). We can skip adding that check, if the lint warning mentioned in that wiki page never fires.

[MatrixFrog]

Sigh. The Arguments object's toString() returns [object Object] (not [object Arguments]) on IE8.

[MatrixFrog]

Discussed with @concavelenz and we agree the simplest path is when creating an iterator from an object, just assume the object is "array like". The downside is that incorrect code may actually run instead of throwing errors, but if users enable typechecking this is unlikely to happen.