Try in traceur repl console.log(Array.isArray(e)) and open console in 6to5 repl. See caveats.

I don't believe Natives can be subclassed yet, unfortunately this is a platform limitation.

In theory some hacks could exist to pseudo implement 1 level of inheritance by fetching the native from another realm and mutating it, but this really is a slippery slope.

class FooArray extends Array {
  constructor(a) {
    super(a);
  }
}
var e = new FooArray();
e.push('bar');
console.log(e);
console.log(e.length);
console.log(e instanceof Array);
console.log(Array.isArray(e))

traceur:
["bar"]
1
true
false
6to5:
{"0":"bar","length":1}
1
true
false

Duplicate? #570

See Caveats

Leaving this here for anyone Googling this error message:

subclass_string.js

class MyString extends String {
    constructor (value) {
        super(value)
    }
}

let test = new MyString('foo');

test += 'bar';
console.log(test);

result

$ babel-node subclass_string.js

subclass_string.js:9
test += 'bar';
    ^
TypeError: String.prototype.valueOf is not generic