Upgrade terser & enable parallel minification in wsl (#7857)

packages/react-scripts/config/webpack.config.js

@@ -9,7 +9,6 @@
 'use strict';
 
 const fs = require('fs');
-const isWsl = require('is-wsl');
 const path = require('path');
 const webpack = require('webpack');
 const resolve = require('resolve');
@@ -255,9 +254,7 @@ module.exports = function(webpackEnv) {
           },
           // Use multi-process parallel running to improve the build speed
           // Default number of concurrent runs: os.cpus().length - 1
-          // Disabled on WSL (Windows Subsystem for Linux) due to an issue with Terser
-          // https://github.com/webpack-contrib/terser-webpack-plugin/issues/21
-          parallel: !isWsl,
+          parallel: true,
           // Enable file caching
           cache: true,
           sourceMap: shouldUseSourceMap,

packages/react-scripts/package.json

@@ -54,7 +54,6 @@
     "fs-extra": "7.0.1",
     "html-webpack-plugin": "4.0.0-beta.5",
     "identity-obj-proxy": "3.0.0",
-    "is-wsl": "^1.1.0",
     "jest": "24.9.0",
     "jest-environment-jsdom-fourteen": "0.1.0",
     "jest-resolve": "24.9.0",
@@ -74,7 +73,7 @@
     "sass-loader": "7.2.0",
     "semver": "6.3.0",
     "style-loader": "1.0.0",
-    "terser-webpack-plugin": "1.4.1",
+    "terser-webpack-plugin": "2.1.3",
     "ts-pnp": "1.1.4",
     "url-loader": "2.1.0",
     "webpack": "4.41.0",