This is a weird one since it runs fine locally but fails on builders. It appears to have problems finding chopsui elements and the redux.js file (again, only on the builder. Workstation runs this fine).

I added some more verbose logging to the wct.go harness and re-ran it via led (changes here: https://chromium-review.googlesource.com/c/infra/infra/+/1124941 ). Relevant log messages:

[I2018-07-03T09:56:41.940616-07:00 7023 0 wct.go:147] [DEBUG] -> [{"method":"Log.entryAdded","params":{"entry":{"source":"network","level":"error","text":"Failed to load resource: the server responded with a status of 404 (Not Found)","timestamp":1530636992046.3,"url":"http://127.0.0.1:38420/node_modules/redux/dist/redux.js","networkRequestId":"7660.81"}}}]
[I2018-07-03T09:56:41.940719-07:00 7023 0 wct.go:147] [DEBUG] -> [{"method":"Log.entryAdded","params":{"entry":{"source":"network","level":"error","text":"Failed to load resource: the server responded with a status of 404 (Not Found)","timestamp":1530636992047.21,"url":"http://127.0.0.1:38420/bower_components/chopsui/chops-header.html","networkRequestId":"7660.85"}}}]
[I2018-07-03T09:56:41.940771-07:00 7023 0 wct.go:147] [DEBUG] -> [{"method":"Log.entryAdded","params":{"entry":{"source":"network","level":"error","text":"Failed to load resource: the server responded with a status of 404 (Not Found)","timestamp":1530636992089.71,"url":"http://127.0.0.1:38420/bower_components/chopsui/chops-timestamp.html","networkRequestId":"7660.116"}}}]
[I2018-07-03T09:56:41.940807-07:00 7023 0 wct.go:147] [DEBUG] -> [{"method":"Log.entryAdded","params":{"entry":{"source":"network","level":"error","text":"Failed to load resource: the server responded with a status of 404 (Not Found)","timestamp":1530636992098.07,"url":"http://127.0.0.1:38420/bower_components/chopsui/chops-dialog.html","networkRequestId":"7660.122"}}}]
[I2018-07-03T09:56:41.940848-07:00 7023 0 wct.go:147] [DEBUG] -> [{"method":"Log.entryAdded","params":{"entry":{"source":"network","level":"error","text":"Failed to load resource: the server responded with a status of 404 (Not Found)","timestamp":1530636992127.27,"url":"http://127.0.0.1:38420/bower_components/chopsui/chops-button.html","networkRequestId":"7660.148"}}}]

Complete logs:
https://logs.chromium.org/v/?s=infra%2Fled%2Fseanmccullough_google.com%2Fa37a27532283050ae3b6ad61fa5ca6b9ac3cdaf47ee15eeaf498c567136a414b%2F%2B%2Fsteps%2FMonorail_WCT_Tests%2F0%2Fstdout