Broccoli Builder ran into an error with `broccoli-persistent-filter:Babel > [Babel: ember-cli-fastboot]` plugin. 💥

Using `ember-cli-babel` `6.12.0` / `6.14.1` / `6.17.0` to build an Ember 3.5 app with FastBoot, everything works fine. However, when I try to debug the FastBoot requests by starting the app with `node --inspect` as [documented here](https://github.com/ember-fastboot/ember-cli-fastboot#developer-tools), the build fails.

Tested using `node` `8.11.3` and `8.12.0`:

```
$ node --inspect ./node_modules/.bin/ember s
Debugger listening on ws://127.0.0.1:9229/67d1641a-fd42-4b42-8eff-1128e75cfb9c
For help see https://nodejs.org/en/docs/inspector
Could not start watchman
Visit https://ember-cli.com/user-guide/#watchman for more info.
Warning: failed to stat /var/www/app/tmp/broccoli_merge_trees-input_base_path-0386spy4.tmp/0
Debugger listening on ws://127.0.0.1:43210/59e7e16d-9937-4614-b44d-e797b4e01834
For help see https://nodejs.org/en/docs/inspector
Debugger listening on ws://127.0.0.1:43211/3d1c5f27-8e23-4dfb-8d98-56b83c710794
For help see https://nodejs.org/en/docs/inspector
Debugger listening on ws://127.0.0.1:43212/ee86c720-a1af-4500-9c49-fe026a0f855f
For help see https://nodejs.org/en/docs/inspector
Starting inspector on 127.0.0.1:43210 failed: address already in use
Starting inspector on 127.0.0.1:43211 failed: address already in use
Starting inspector on 127.0.0.1:43212 failed: address already in use
Build Error (broccoli-persistent-filter:Babel > [Babel: ember-cli-fastboot]) in ember-cli-fastboot/instance-initializers/clear-double-boot.js

Worker terminated unexpectedly

Stack Trace and Error Report: /tmp/error.dump.d0a52049fa710c3edc78a1199ac0e92d.log
```

`error.dump.log`:
```
ENV Summary:

TIME: Thu Nov 15 2018 10:32:51 GMT+0100 (CET)
TITLE: ember
ARGV:
- /home/redsandro/.nvm/versions/node/v8.11.3/bin/node
- /var/www/app/node_modules/.bin/ember
- s
EXEC_PATH: /home/redsandro/.nvm/versions/node/v8.11.3/bin/node
TMPDIR: /tmp
SHELL: /bin/bash
PATH:
- /home/redsandro/.nvm/versions/node/v8.11.3/bin
- /home/redsandro/bin
- /usr/local/sbin
- /usr/local/bin
- /usr/sbin
- /usr/bin
- /sbin
- /bin
- /usr/games
- /usr/local/games
PLATFORM: linux x64
FREEMEM: 9321517056
TOTALMEM: 16784859136
UPTIME: 685
LOADAVG: 1.05810546875,0.8154296875,0.55859375
CPUS:
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1908
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1891
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1977
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1729
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1825
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1831
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1708
- Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz - 1721
ENDIANNESS: LE
VERSIONS:
- ares: 1.10.1-DEV
- cldr: 32.0
- http_parser: 2.8.0
- icu: 60.1
- modules: 57
- napi: 3
- nghttp2: 1.32.0
- node: 8.11.3
- openssl: 1.0.2o
- tz: 2017c
- unicode: 10.0
- uv: 1.19.1
- v8: 6.2.414.54
- zlib: 1.2.11

ERROR Summary:

- broccoliBuilderErrorStack: Error: Worker terminated unexpectedly
at ChildProcess. (/var/www/app/node_modules/ember-cli-fastboot/node_modules/workerpool/lib/WorkerHandler.js:181:17)
at emitTwo (events.js:126:13)
at ChildProcess.emit (events.js:214:7)
at Process.ChildProcess._handle.onexit (internal/child_process.js:198:12)
- codeFrame: Worker terminated unexpectedly
- errorMessage: Build Canceled: Broccoli Builder ran into an error with `broccoli-persistent-filter:Babel > [Babel: ember-cli-fastboot]` plugin. 💥
Worker terminated unexpectedly
- errorType: Build Error
- location:
- column: [undefined]
- file: ember-cli-fastboot/instance-initializers/clear-double-boot.js
- line: [undefined]
- treeDir: /var/www/app/tmp/broccoli_persistent_filterbabel__babel_ember_cli_fastboot-input_base_path-pjCTzMzJ.tmp
- message: Build Canceled: Broccoli Builder ran into an error with `broccoli-persistent-filter:Babel > [Babel: ember-cli-fastboot]` plugin. 💥
Worker terminated unexpectedly
- name: Error
- nodeAnnotation: Babel: ember-cli-fastboot
- nodeName: broccoli-persistent-filter:Babel > [Babel: ember-cli-fastboot]
- originalErrorMessage: Worker terminated unexpectedly
- stack: Error: Worker terminated unexpectedly
at ChildProcess. (/var/www/app/node_modules/ember-cli-fastboot/node_modules/workerpool/lib/WorkerHandler.js:181:17)
at emitTwo (events.js:126:13)
at ChildProcess.emit (events.js:214:7)
at Process.ChildProcess._handle.onexit (internal/child_process.js:198:12)
```

I can't seen to comprehend what the problem could be.

---

Originally reported at https://github.com/ember-fastboot/ember-cli-fastboot/issues/645