Warnings when running `yarn amo:dev`

There are a couple of warnings that now appear in the console:

[0] {
[0]   message: "DefinePlugin\nConflicting values for 'process.env.NODE_ENV'",
[0]   details: `'"production"' !== '"development"'`,
[0]   stack: 'Error: DefinePlugin\n' +
[0]     "Conflicting values for 'process.env.NODE_ENV'\n" +
[0]     '    at /Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/webpack/lib/DefinePlugin.js:573:24\n' +
[0]     '    at Array.forEach (<anonymous>)\n' +
[0]     '    at walkDefinitionsForValues (/Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/webpack/lib/DefinePlugin.js:564:31)\n' +
[0]     '    at /Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/webpack/lib/DefinePlugin.js:591:5\n' +
[0]     '    at _next28 (eval at create (/Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/tapable/lib/HookCodeFactory.js:19:10), <anonymous>:44:1)\n' +
[0]     '    at _next6 (eval at create (/Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/tapable/lib/HookCodeFactory.js:19:10), <anonymous>:97:1)\n' +
[0]     '    at Hook.eval (eval at create (/Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/tapable/lib/HookCodeFactory.js:19:10), <anonymous>:113:1)\n' +
[0]     '    at Hook.CALL_DELEGATE [as _call] (/Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/tapable/lib/Hook.js:14:14)\n' +
[0]     '    at Compiler.newCompilation (/Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/webpack/lib/Compiler.js:1122:26)\n' +
[0]     '    at /Users/bsilverberg/Documents/gitRepos/addons-frontend/node_modules/webpack/lib/Compiler.js:1166:29'
[0] }As well as:```

[0] WARNING in DefinePlugin [0] Conflicting values for 'process.env.NODE_ENV' [0] [0] 1 warning has detailed information that is not shown. [0] Use 'stats.errorDetails: true' resp. '--stats-error-details' to show it.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/AMOENG-404)

Likely caused by the webpack upgrade in https://github.com/mozilla/addons/issues/14531 but I don't think the underlying problem is new.

This is caused by: https://github.com/mozilla/addons-frontend/blob/5518fc1091241849f1f90634fe5d8f5d3cce2688/webpack-common.js#L132

Which has been there for a very long time, but I'm not sure why...

This issue has been automatically marked as stale because it has not had recent activity. If you think this bug should stay open, please comment on the issue with further details. Thank you for your contributions.

Old Jira Ticket: https://mozilla-hub.atlassian.net/browse/ADDFRNT-32