this assignment is inserted before 'use strict'

My ES6 code:

'use strict'

setTimeout(() => {
    this.happened = true
})

...is converted by Buble@0.10.6 into:

var this$1 = this;
'use strict'

setTimeout(function () {
    this$1.happened = true
})

However, I expect:

'use strict'
var this$1 = this;

setTimeout(function () {
    this$1.happened = true
})