Error: Cannot find module '@babel/plugin-proposal-private-methods'

Question

When checking out of the latest master, I receive the following error when I try to build Webpack. The only noticeable change was a single gem that was installed during the required bundle install that I ran post-pulling & fetching the latest master. However, I didn't notice the Webpack Error straight away, and so am at a loss as to what the Gem was. I assume I'm missing a dependency of that Gem or similar. However, I'm not well versed in Webpack and would appreciate some guidance and what the next steps should be.

This is the root error:

    Module build failed (from ./node_modules/babel-loader/lib/index.js):
    Error: Cannot find module '@babel/plugin-proposal-private-methods' from '/Users/user/pair'
    at Function.module.exports [as sync] (/Users/user/pair/node_modules/resolve/lib/sync.js:74:15)
    at resolveStandardizedName (/Users/user/pair/node_modules/@babel/core/lib/config/files/plugins.js:101:31)
    at resolvePlugin (/Users/user/pair/node_modules/@babel/core/lib/config/files/plugins.js:54:10)
    at loadPlugin (/Users/user/pair/node_modules/@babel/core/lib/config/files/plugins.js:62:20)
    at createDescriptor (/Users/user/pair/node_modules/@babel/core/lib/config/config-descriptors.js:154:9)
    at /Users/user/pair/node_modules/@babel/core/lib/config/config-descriptors.js:109:50
    at Array.map (<anonymous>)
    at createDescriptors (/Users/user/pair/node_modules/@babel/core/lib/config/config-descriptors.js:109:29)
    at createPluginDescriptors (/Users/user/pair/node_modules/@babel/core/lib/config/config-descriptors.js:105:10)
    at plugins (/Users/user/pair/node_modules/@babel/core/lib/config/config-descriptors.js:40:19)