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)
```

Run the reproduction steps in the issue from the tape repository, comparing yarn test with dist/index.modern.js, dist/index.module.js, and dist/index.cjs in test/src/*.js. Start by locating where the failing Cannot read property 'value' of undefined error occurs and compare the generated bundles. Done means the tests pass when each of the three dist entry points is substituted.

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