fix: improve isSouceMap check

[alexander-akait]

fixes #283

[joshwiens]

@sokra - He has a point about the error output, i've run into this before & per the comment in the original issue this is super hard to debug if you don't know what you are looking for

[nehaabrol87]

@sokra With what version of webpack 4 is this plugin available ? When mode = 'production' , webpack 4 automatically includes UglifyJsPlugin . Ever since I upgraded my preact project to use webpack 4 , my development build was working fine but some react beatutiful dnd modules were being rendered as undefined only in production.

After going down a lot of rabbit holes , I figured setting mode = development on production webpack config works but of course I still do not understand if the real problem is webpack 4 or preact-compat not working well with any of the automatically included plugins in webpack 4 -

Plugins automatically available with webpack 4

Provides process.env.NODE_ENV with value production. Enables FlagDependencyUsagePlugin, FlagIncludedChunksPlugin, ModuleConcatenationPlugin, NoEmitOnErrorsPlugin, OccurrenceOrderPlugin, SideEffectsFlagPlugin and UglifyJsPlugin

Setting that works in my project and does not render undefined for react-beautiful-dnd modules

mode = 'development'
and adding
optimization: {
namedModules: true, // NamedModulesPlugin()
splitChunks: { // CommonsChunkPlugin()
name: 'vendor',
minChunks: 2
},
noEmitOnErrors: true, // NoEmitOnErrorsPlugin
concatenateModules: true
},

NOTE: Long story short , I still do not understand if the real problem is webpack 4 or preact-compat not working well with any of the automatically included plugins in webpack 4.
I am going too notify preact-compat folks as well.

Versions
webpack@4.8.1
preact-compat@3.18.0