debian/build_modules/webpack-stream has been modified because webapck waits
for mkdirp-1 API whole memory-fs looks like a mkdirp-0.53:

--- a/debian/build_modules/webpack-stream/index.js
+++ b/debian/build_modules/webpack-stream/index.js
@@ -172,6 +172,16 @@ module.exports = function (options, wp, done) {
       }

       var fs = compiler.outputFileSystem = new MemoryFileSystem();
+      // Wrapper around mkdirp (webpack waits for mkdirp-1 API)
+      fs.realMkdrip = fs.mkdirp;
+      compiler.outputFileSystem.mkdirp = function(outputPath) {
+        return new Promise((resolve,reject) => {
+         fs.realMkdrip(outputPath, function(er) {
+           if (er) return reject(er);
+           return resolve();
+         })
+       })
+      }

       var afterEmitPlugin = compiler.hooks
         // Webpack 4
