How would a meteor file have become slightly corrupted

I’m running on meteor 1.8-rc15 and have been for a week or so (working for all that time). Today, when I ran meteor npm start I got an error:

var toInteger = require('./_to-integer'):
W20180924-11:51:28.817(-4)? (STDERR) SyntaxError: Unexpected token :
W20180924-11:51:28.818(-4)? (STDERR)     at createScript (vm.js:80:10)
W20180924-11:51:28.818(-4)? (STDERR)     at Object.runInThisContext (vm.js:139:10)
W20180924-11:51:28.818(-4)? (STDERR)     at Module._compile (module.js:616:28)
W20180924-11:51:28.818(-4)? (STDERR)     at Object.Module._extensions..js (module.js:663:10)
W20180924-11:51:28.819(-4)? (STDERR)     at Module.load (module.js:565:32)
W20180924-11:51:28.819(-4)? (STDERR)     at tryModuleLoad (module.js:505:12)
W20180924-11:51:28.819(-4)? (STDERR)     at Function.Module._load (module.js:497:3)
W20180924-11:51:28.819(-4)? (STDERR)     at Module.require (module.js:596:17)
W20180924-11:51:28.819(-4)? (STDERR)     at require (internal/module.js:11:18)

The problem relates to the below file, which comes from the ecmascript-runtime-server package, and internally uses the core-js npm package (version 2.5.1) I checked, and 2.5.1 of this package uses ; as is correct, but mine uses : which is obviously incorrect.

How could I have corrupted just this file?