This bug is reproducible in our workspace (linux ubuntu dev for mobile UC & chrome).
The rootcause is in appcache.cc.
Since some unkown (for me) version, devtools start sending formatter_worker.js request with both query string and ref string.
But even the latest chromium code in appcache.cc doesn't strip out query parts.

A simple patch is suggested in attachment. ^_

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I think we send the query string to the formatter_worker intentionally. Can you provide us a repro case (e.g. you can use jsfiddle or any other code snippet site)?

Yes, maybe the root cause is in 'serve_rev'

the request url is
chrome-devtools://devtools/remote/serve_rev/@e76cc6b93d194dac46a61d88327a96a0d5cf9894/formatter_worker.js?remoteFrontend=true&dockSide=undocked

This url fail in appcache.cc's key lookup, causing the formatter worker fail to load.

It happens when debugging webpage for mobile browser with pc chrome.

If chrome send query string intentionally. The `serve_rev/' should not put formatter_worker.js into Applicache Cache manifest file in response.

Or at lease add a network fallback section:

# Resources that require the user to be online.
NETWORK:
*

One can still use format_worker.js without any code modification as below:

1. Connect your phone's inspect as usual.
2. When format worker fail to load, cut off network
3. Appcache will be disbled(the inspector front-end will be usable with help of web cache)

This problem is especially prevalent for developers in China (e.g. the OP from UC Browser), where reaching formatter_worker appears to be more problematic.

I reproduce this bug too.
1. open chrome for android (Chrome (59.0.3071.92))
2. visit https://m.aliexpress.com/
3. try to format sw.js in RI of chrome 58.0.3029.110 (64-bit) 
4. nothing happened.
 

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Ping for chenwilliam@; any update here please? 

Sorry for the long delay. There's a fix in flight that will fix this and linked bug which have the same underlying issue:
https://chromium-review.googlesource.com/c/chromium/tools/chrome-devtools-frontend/+/762143

finally.