It took me close to one day to identify zone.js 0.7 as the root cause for this problem and identify a downgrade to 0.6 as possible workaround. I very much hope that someone can pick this up from here and fix the issue so that no one else has to go through the same experience:

To reproduce:

1. clone, build and run the demo from https://github.com/nolanlawson/hello-electron-with-pouchdb as documents
   -> npm install, npm start, to this point everything should work fine (all four database connections work)

2. include zone.js version 0.6.26 into the project:
   a) add '"zone.js": "0.6.26" to dependencies in package.json
   b) add '<script src="node_modules/zone.js/dist/zone.js"></script>' to in index.hml
   -> npm install, npm start, everything should still work fine

3. update zone.js to 0.7.4:
   -> npm install, npm start, now you get "Error for LevelDB"

(when removing the .catch statement, I can obtain the actual error message as:

Error: Could not locate the bindings file. Tried:
→ /Users/norbert/repo/hello-electron-with-pouchdb/build/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/build/Debug/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/build/Release/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/out/Debug/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/Debug/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/out/Release/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/Release/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/build/default/leveldown.node
→ /Users/norbert/repo/hello-electron-with-pouchdb/compiled/6.5.0/darwin/x64/leveldown.node: unable to import leveldown
at new Error (native)
at requireLeveldown (/Users/norbert/repo/hello-electron-with-pouchdb/node_modules/pouchdb/lib/index.js:5112:12) []
at PouchDB.LevelDownPouch (/Users/norbert/repo/hello-electron-with-pouchdb/node_modules/pouchdb/lib/index.js:5312:17) []
at new PouchDB (/Users/norbert/repo/hello-electron-with-pouchdb/node_modules/pouchdb/lib/index.js:2701:34) []