upgrade to webpack 4

.babelrc.js

@@ -23,7 +23,8 @@ module.exports = {
             "ie >= 10",
             "ios >= 8"
           ]
-        }
+        },
+        modules: 'commonjs' //This causes Babel to transpile ES modules into CJS modules before Webpack sees them, which prevents Webpack from creating getters, which fixes Sinon.(Ref: https://github.com/webpack/webpack/issues/6979)
       }
     ]
   ],

gulpHelpers.js

@@ -127,7 +127,7 @@ module.exports = {
   nameModules: function(externalModules) {
     var modules = this.getModules(externalModules);
     return through.obj(function(file, enc, done) {
-      file.named = modules[file.path] ? modules[file.path] : 'prebid';
+      file.named = modules[file.path] ? modules[file.path] : 'prebid-core';
       this.push(file);
       done();
     })

gulpfile.js

@@ -22,6 +22,7 @@ var shell = require('gulp-shell');
 var eslint = require('gulp-eslint');
 var gulpif = require('gulp-if');
 var sourcemaps = require('gulp-sourcemaps');
+var filter = require('gulp-filter');
 var through = require('through2');
 var fs = require('fs');
 var jsEscape = require('gulp-js-escape');
@@ -66,13 +67,13 @@ function lint(done) {
   }
   const isFixed = function(file) {
     return file.eslint != null && file.eslint.fixed;
-  }
+  };
   return gulp.src(['src/**/*.js', 'modules/**/*.js', 'test/**/*.js'], {base: './'})
     .pipe(gulpif(argv.nolintfix, eslint(), eslint({fix: true})))
     .pipe(eslint.format('stylish'))
     .pipe(eslint.failAfterError())
     .pipe(gulpif(isFixed, gulp.dest('./')));
-};
+}
 
 // View the code coverage report in the browser.
 function viewCoverage(done) {
@@ -86,7 +87,7 @@ function viewCoverage(done) {
   });
   opens('http://' + mylocalhost + ':' + coveragePort);
   done();
-};
+}
 
 viewCoverage.displayName = 'view-coverage';
 
@@ -115,36 +116,32 @@ function watch(done) {
   done();
 };
 
-function makeDevpackPkg() {
+function makeWebpackPkg(dev) {
   var cloned = _.cloneDeep(webpackConfig);
-  cloned.devtool = 'source-map';
   var externalModules = helpers.getArgModules();
 
-  const analyticsSources = helpers.getAnalyticsSources();
-  const moduleSources = helpers.getModulePaths(externalModules);
-
-  return gulp.src([].concat(moduleSources, analyticsSources, 'src/prebid.js'))
-    .pipe(helpers.nameModules(externalModules))
-    .pipe(webpackStream(cloned, webpack))
-    .pipe(gulp.dest('build/dev'))
-    .pipe(connect.reload());
-}
-
-function makeWebpackPkg() {
-  var cloned = _.cloneDeep(webpackConfig);
-  delete cloned.devtool;
-
-  var externalModules = helpers.getArgModules();
+  if (!dev) {
+    delete cloned.devtool;
+    cloned.mode = 'production';
+  }
 
   const analyticsSources = helpers.getAnalyticsSources();
   const moduleSources = helpers.getModulePaths(externalModules);
 
+  var coreFilter = filter(['**/prebid-core.js', '**/prebid-shared.js'], {restore: true});
+
   return gulp.src([].concat(moduleSources, analyticsSources, 'src/prebid.js'))
     .pipe(helpers.nameModules(externalModules))
     .pipe(webpackStream(cloned, webpack))
-    .pipe(uglify())
-    .pipe(gulpif(file => file.basename === 'prebid-core.js', header(banner, { prebid: prebid })))
-    .pipe(gulp.dest('build/dist'));
+    .pipe(coreFilter)
+    .pipe(sourcemaps.init({loadMaps: true}))
+    .pipe(concat('prebid-core.js'))
+    .pipe(sourcemaps.write())
+    .pipe(coreFilter.restore)
+    .pipe(gulpif(!dev, uglify()))
+    .pipe(gulpif(file => (!dev && file.basename === 'prebid-core.js'), header(banner, { prebid: prebid })))
+    .pipe(gulp.dest(dev ? 'build/dev' : 'build/dist'))
+    .pipe(connect.reload());
 }
 
 function gulpBundle(dev) {
@@ -202,7 +199,7 @@ function bundle(dev, moduleArr) {
       global: prebid.globalVarName
     }
     )))
-    .pipe(gulpif(dev, sourcemaps.write('.')));
+    .pipe(gulpif(dev, sourcemaps.write()));
 }
 
 // Run the unit tests.
@@ -298,8 +295,8 @@ gulp.task(clean);
 
 gulp.task(escapePostbidConfig);
 
-gulp.task('build-bundle-dev', gulp.series(makeDevpackPkg, gulpBundle.bind(null, true)));
-gulp.task('build-bundle-prod', gulp.series(makeWebpackPkg, gulpBundle.bind(null, false)));
+gulp.task('build-bundle-dev', gulp.series(makeWebpackPkg.bind(null, true), gulpBundle.bind(null, true)));
+gulp.task('build-bundle-prod', gulp.series(makeWebpackPkg.bind(null, false), gulpBundle.bind(null, false)));
 
 // public tasks (dependencies are needed for each task since they can be ran on their own)
 gulp.task('test', gulp.series(clean, lint, test));
@@ -313,7 +310,7 @@ gulp.task('build', gulp.series(clean, 'build-bundle-prod'));
 gulp.task('build-postbid', gulp.series(escapePostbidConfig, buildPostbid));
 
 gulp.task('serve', gulp.series(clean, lint, gulp.parallel('build-bundle-dev', watch, test)));
-gulp.task('default', gulp.series(clean, makeWebpackPkg));
+gulp.task('default', gulp.series(clean, makeWebpackPkg.bind(null, true)));
 
 gulp.task('e2e-test', gulp.series(clean, setupE2e, gulp.parallel('build-bundle-dev', watch), test))
 // other tasks

karma.conf.maker.js

@@ -11,9 +11,9 @@ function newWebpackConfig(codeCoverage) {
   // Make a clone here because we plan on mutating this object, and don't want parallel tasks to trample each other.
   var webpackConfig = _.cloneDeep(webpackConf);
 
-  // remove optimize plugin for tests
+  // remove plugin for tests
   webpackConfig.plugins.pop()
-
+  webpackConfig.optimization = {};
   webpackConfig.devtool = 'inline-source-map';
 
   if (codeCoverage) {