Upgrade to webpack v3.4.0

Upgrading to webpack v3 promises several improvements including:

• scope hoisting (removing many unnecessary closures and improving runtime performance)
• build performance improvements

While it works pretty much out of the box, many of the plugins we use have not yet been updated to recognize the v3.x semver as a valid peer dependency. As a result, yarn check will fail until we update the following packages:

• babel-loader
• stats-webpack-plugin
• eslint-import-resolver-webpack
• karma-webpack
• webpack-dev-server
• worker-loader
• webpack-dev-middleware

Closes #33902 (closed)