async/await -> using array method yields error

Hello, I ran into the following issue:

1. code:
```js
const testarray = [
{id: 1, name: 'foo'},
{id: 2, name: 'bar'}
]
const doarray = [2]

const test = async () => {
for (let i=0; i < doarray.length; i++) {
const a = testarray.findIndex(x => x.id === doarray[i])
await Promise.resolve(console.log(a))
}
}
test()
```
result: Error: Cannot use keyword 'await' outside an async function.

2. code:
```js
const testarray = [
{id: 1, name: 'foo'},
{id: 2, name: 'bar'}
]
const doarray = [2]

const test = async () => {
const geta = i => testarray.findIndex(x => x.id === doarray[i])

for (let i=0; i < doarray.length; i++) {
const a = geta(i)
await Promise.resolve(console.log(a))
}
}
test()
```
result: compiles fine.

This was encountered using Rollup 2.21.0 and @rollup/plugin-buble 0.21.3 (Windows 10), with:
```js
transforms: {asyncAwait: false}
```

Reproduce both JavaScript snippets with Rollup 2.21.0 and @rollup/plugin-buble 0.21.3 on Windows 10, using transforms: {asyncAwait: false}. Compare the inline findIndex callback with the helper-function form; done means the first form compiles without the reported “await outside an async function” error.

Written by the indexing model from the issue text.