[polyserve] Serving extremely slow on sinon package bundle

[TimvdLippe]

Serving the sinon.js bundle with polyserve is extremely slow (>12 seconds).

To reproduce:

1. Check out branch fix-audit-wct-browser-legacy
2. Install the dependencies per usual instructions
3. Run polymer serve in the root of this repository
4. Open http://127.0.0.1:8081/components/polymer-tools/packages/wct-browser-legacy/node_modules/sinon/pkg/sinon.js (check your port)
5. Observe that loading this particular file is extremely slow

Even when running with polymer serve --compile never, it remains slow. Note that other files (for example http://127.0.0.1:8081/components/polymer-tools/packages/wct-browser-legacy/node_modules/sinon/History.md) are instantly loading. Also note that after you refresh the file, it is instant again.

This leads me to believe that there is a performance issue with reading/writing ASTs to files on first load.

This issue is blocking the migration of #533

[usergenic]

polyserve and polymer serve should probably have an exclusion list for middleware that just enables pass-thru of the files which could sidestep the issue. I'll have a go at that feature; its come up several times before and I think there's even some hardwired cruft in polyserve that already passes-thru a couple files by string-matching.

[TimvdLippe]

@usergenic Awesome, thanks for the pointer! I went ahead and opened #543, which fixed the issue for me. Back to tracking the WCT issue now.