Use memory-fs everywhere

Author: geowarin

test/integration.spec.js

@@ -4,7 +4,12 @@ const webpack = require('webpack');
 const FriendlyErrorsWebpackPlugin = require('../src/friendly-errors-plugin');
 const MemoryFileSystem = require('memory-fs');
 
-const syncWebpack = deasync(webpack);
+const syncWebpack = deasync(function(config, fn) {
+  const compiler = webpack(config);
+  compiler.outputFileSystem = new MemoryFileSystem();
+  compiler.run(fn);
+  return compiler;
+});
 
 // Applys plugin directly to compiler to support `MultiCompiler` tests.
 const syncWebpackWithPlugin = deasync(function(config, fn) {