Cannot read property 'value' of undefined

I am getting some interestingly erroneous builds for code relying on `AsyncIterable` and `AsyncIterator`. Steps to reproduce:

```sh
git clone https://github.com/async-abstraction/tape
cd tape
yarn
yarn test # This works fine
sed 's/[/][s][r][c][/][i][n][d][e][x][.][j][s]/\/dist\/index.modern.js/g' -i test/src/*.js
yarn test # Works fine too
git reset --hard HEAD
sed 's/[/][s][r][c][/][i][n][d][e][x][.][j][s]/\/dist\/index.module.js/g' -i test/src/*.js
yarn test # BROKEN (Cannot read property 'value' of undefined)
git reset --hard HEAD
sed 's/[/][s][r][c][/][i][n][d][e][x][.][j][s]/\/dist\/index.cjs/g' -i test/src/*.js
yarn test # BROKEN (Cannot read property 'value' of undefined)
```