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The size change of @babel/preset-env-standalone comes from the fact that @babel/plugin-transform-named-capturing-groups is not included in https://github.com/babel/babel/blob/master/packages/babel-standalone/scripts/pluginConfig.json, therefore, it will instead be bundled into @babel/preset-env-standalone instead of @babel/standalone.

Looking into the pluginConfig.json, it seems that even while Stage 0 proposal (function-bind) is included in pluginConfig, a Stage 4 proposal (named-capturing-groups) is not included. I believe it is a bug and would propose to move all notIncludedPlugins

into pluginsConfig.json. We expect a slight size increase of babel.js and decrease of babel-preset-env.js. In all the total size will be reduced since we are not including regexp-tree.

Side note: we may need specify the peerDependencies of @babel/preset-env-standalone to be @babel/standalone.

IIRC, notIncludedPlugins includes all the plugin from preset-env which are not part of the es* or stage-* presets.

Ideally, @babel/standalone should be as small as possible and all the plugins should only be in @babel/preset-env-standalone, but for legacy reasons we can't just move them.
By doing so, users can update preset-env-standalone without having to udpate @babel/standalone when a new plugin is added.
Unless there is a webpack hack to load regexpu-core from the @babel/standalone bundle in @babel/preset-env-standalone, I don't think that there is anything that we can do to remove that duplicated dependency.

Thank you for the explainers!

Unless there is a webpack hack to load regexpu-core from the @babel/standalone bundle in @babel/preset-env-standalone, I don't think that there is anything that we can do to remove that duplicated dependency.

The webpack dll is supposed to offer a runtime require-able libraries for webpack application. But from my previous experience I would rather stay away from it especially when we required cross-version compatibility between babel/standalone and preset-env-standalone.

I guess we have to accept the size change. But maybe we can deprecate plugin usage in babel-standalone? So we can move plugins to preset-env on v8 (not the JS engine).