Babel does not parenthesize yield expressions inside ternary conditional test

[bjouhier]

Bug found in babel 6.16.

Source:

function* foo() {
  return (yield 1) ? 2 : 3;
}
console.log(foo().next());

Transformed:

function* foo() {
  return yield 1 ? 2 : 3;
}
console.log(foo().next());

Parentheses around yield 1 have been dropped!

Steps to reproduce:

# save source as foo.js
$ node foo.js
{ value: 1, done: false } 
$ babel foo.js > bar.js
$ node bar.js
{ value: 2, done: false }

Fix was easy. I'm going to submit a PR.