ES2015 Issue with Template Literal Object > string substitution

[p-bakker]

This test fails, as Template Literal uses the Token.ADD operation to stringify o, whereas it should be using the ToString operations (see last alinea of https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#string_coercion)

var o = {toString: () => "foo", valueOf: () => "bar"};
`${o}` === "foo"

The code that makes it use the ADD operation is located at

rhino/src/org/mozilla/javascript/IRFactory.java

Line 1061 in 05ab4ff

pn = createBinary(Token.ADD, pn, transform(elem));

. Not sure how to best tackle this codewise