compile-devtools is failing

[paulirish]

The yarn compile-devtools script has been failing for the past few days.

Here's two recent builds from master:

PASSING: https://travis-ci.org/GoogleChrome/lighthouse/jobs/351077847#L828
FAILING: https://travis-ci.org/GoogleChrome/lighthouse/jobs/351130910#L826

Both appear to check out the exact same hash of -frontend and -protocol. Which indicates it's only Lighthouse that's changing.

Here's the error:

devtools frontend compilation output:
/tmp/tmp6f4UuQ/front_end/bindings/FileUtils.js:146: WARNING - Property result never defined on EventTarget
    const buffer = event.target.result;
                                ^^^^^^

I'm not really sure why this started cropping up after bc9efba, but a bisect confirmed that this is the commit where it started.

[paulirish]

This event.target.result is coming from filereader's onload event.

I don't see any recent changes to Event, EventTarget happening in our externs, devtools externs, or the closure ones (and here).

and running yarn run closure against latest devtools frontend reports no errors.