safeguard against duplicate on/using calls

[brigand]

My understanding is that calling on/using more than once will cause the second call to influence new instances of the first class it's used on.

This is just a check to identify programmer error.

[brigand]

Just to clarify, the case is something like this:

// example.js 
require('es6-shim');
const traits = require('./es6-traits.min.js');
const {on, using} = traits();

const T1 = {
  foo: () => 'T1'
};

class A extends (on (Object), using (T1)) {
    bar(){
        console.log('A', this.foo());
    }
}

const T2 = {
  foo: () => 'T2'
};

class B extends (on (Object), using (T2)) {
    bar(){
        console.log('B', this.foo());
    }
}

var a = new A();
var b = new B();

a.bar();
b.bar();

Likely expected:

A T1
B T2

Actual:

A T1
B T1

npm i es6-shim && babel example.js | node

[brigand]

After reading the code a couple more times, I now see that this is what Symbol.toStringTag is intended to solve. A bit odd, but functioning.

[gsklee]

@brigand Yep, it's essentially naming the mixins; otherwise, it'll be difficult to tell whether two objects are the same mixin or not.