No support for multiple parallel retrieve_dependencies() calls.

I have more than two files depending on the same files, and that cause the following problem:

Here is the error message:

/home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/code-class.js:73
throw new Error([
^

Error: No support for multiple parallel retrieve_dependencies() calls. Yet a second parallel call is being executed on /home/jehon/src/cryptomedic/www/bower_components/iron-meta/iron-meta.html. Run the second call sequentially instead.
at CodeInfo.retrieve_dependencies (/home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/code-class.js:73:23)
at /home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/workers/dependency.js:22:29
at Array.map (native)
at recurse (/home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/workers/dependency.js:18:10)
at run_worker (/home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/code-class.js:182:17)
at /home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/code-class.js:184:21
at handle_js (/home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/workers/retrievers/requirejs.js:74:9)
at run_worker (/home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/code-class.js:182:17)
at /home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/code-class.js:184:21
at /home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/workers/retrievers/requirejs.js:41:9
at Object.done (/home/jehon/src/offline-listing/node_modules/website-dependency-tree/lib/workers/retrievers/requirejs.js:59:13)
at /home/jehon/src/offline-listing/node_modules/jsdom/lib/jsdom.js:259:18
at _combinedTickCallback (internal/process/next_tick.js:67:7)
at process._tickCallback (internal/process/next_tick.js:98:9)

The fact is that theses files are included from my bower dependendies, and that I can't modify them.

Start with lib/code-class.js at retrieve_dependencies() and trace the calls from lib/workers/dependency.js, using the reported stack trace as the entry point. Reproduce the failure with multiple files sharing dependencies such as the listed bower components. Done means parallel retrieval no longer throws the reported error.

Written by the indexing model from the issue text.